Oxide semiconductor transistor

ABSTRACT

A display device includes a driving transistor and an organic EL element. The driving transistor includes an oxide semiconductor layer; a first gate electrode that region overlapping the oxide semiconductor layer; a first insulting layer between the first gate electrode and the oxide semiconductor layer; a second gate electrode that includes a region overlapping the oxide semiconductor layer and the first gate electrode; a second insulating layer between the second gate electrode and the oxide semiconductor layer; and a first and a second transparent conductive layer that are provided between the oxide semiconductor layer and the first insulating layer and each include a region contacting the oxide semiconductor layer. The organic EL element includes a first electrode; a second electrode; a light emitting layer between the first electrode and the second electrode; and an electron transfer layer between the light emitting layer and the first electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/721,271 filed on Sep. 29, 2017, which is based upon and claims thebenefit of priority from the prior Japanese Patent Application No.2017-107278 filed on May 31, 2017 the entire contents of all of whichare incorporated herein by reference.

FIELD

The present invention relates to a structure of a display device. Anembodiment of the present invention relates to a structure of atransistor and a display element included in a pixel of a displaydevice.

BACKGROUND

An active matrix display device includes a display element and atransistor driving the display element that are provided in each ofpixels thereof. Usable as the display element is a liquid crystalelement including a pair of electrodes and a liquid crystal layerprovided between the pair of electrodes or an organicelectroluminescence element (hereinafter, referred to as an “organic ELelement”) including a cathode electrode, an anode electrode and a layerthat is provided between the cathode electrode and the anode electrodeand contains an organic electroluminescence material. The transistor isformed of an amorphous silicon semiconductor or a polycrystallinesilicon semiconductor. Recently, a thin film transistor formed of anoxide semiconductor is also used.

For example, a display device including an organic EL element and adriving transistor that includes a semiconductor layer formed ofsilicon, a gate insulting layer and a gate electrode is disclosed (e.g.,Japanese Laid-Open Patent Publication No. 2007-053286). Also, a displaydevice including an organic EL element and a transistor driving theorganic EL element that are integrally formed of an oxide semiconductoris disclosed (e.g., Japanese Laid-Open Patent Publication No.2014-154382).

SUMMARY

A display device in an embodiment according to the present inventionincludes a substrate; and a plurality of pixels provided on thesubstrate. The plurality of pixels each include a driving transistor andan organic EL element electrically connected with the drivingtransistor. The driving transistor includes an oxide semiconductorlayer; a first gate electrode including a region overlapping the oxidesemiconductor layer, the first gate electrode being provided on asurface of the oxide semiconductor layer facing the substrate; a firstinsulating layer provided between the first gate electrode and the oxidesemiconductor layer; a second gate electrode including a regionoverlapping the oxide semiconductor layer and the first gate electrode,the second gate electrode being provided on a surface of the oxidesemiconductor layer opposite to the surface facing the substrate; asecond insulating layer provided between the second gate electrode andthe oxide semiconductor layer; and a first transparent conductive layerand a second transparent conductive layer provided between the oxidesemiconductor layer and the first insulating layer, the firsttransparent conductive layer and the second transparent conductive layereach including a region in contact with the oxide semiconductor layer.The organic EL element includes a light-transmissive first electrode; asecond electrode provided to face the first electrode; a light emittinglayer provided between the first electrode and the second electrode; andan electron transfer layer provided between the light emitting layer andthe first electrode. The first electrode is continuous from the firsttransparent conductive layer.

A display device in an embodiment according to the present inventionincludes a substrate; and a plurality of pixels provided on thesubstrate. The plurality of pixels each include a driving transistor andan organic EL element electrically connected with the drivingtransistor. The driving transistor includes an oxide semiconductorlayer; a first gate electrode including a region overlapping the oxidesemiconductor layer, the first gate electrode being provided on asurface of the oxide semiconductor layer facing the substrate; a firstinsulating layer provided between the first gate electrode and the oxidesemiconductor layer; a second gate electrode including a regionoverlapping the oxide semiconductor layer and the first gate electrode,the second gate electrode being provided on a surface of the oxidesemiconductor layer opposite to the surface facing the substrate; and asecond insulating layer provided between the second gate electrode andthe oxide semiconductor layer. The organic EL element includes alight-transmissive first electrode; a second electrode provided to facethe first electrode; a light emitting layer provided between the firstelectrode and the second electrode; and an electron transfer layerprovided between the light emitting layer and the first electrode. Thefirst electrode is continuous from the first transparent conductivelayer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing a structure of a transistor inan embodiment according to the present invention;

FIG. 2A shows a method for producing the transistor in an embodimentaccording to the present invention, and shows a stage of forming a firstgate electrode;

FIG. 2B shows the method for producing the transistor in an embodimentaccording to the present invention, and shows a stage of forming a firstinsulating layer, a transparent conductive film, and an oxidesemiconductor layer;

FIG. 3A shows the method for producing the transistor in an embodimentaccording to the present invention, and shows a stage of exposureperformed by use of a multi-gradation photomask;

FIG. 3B shows the method for producing the transistor in an embodimentaccording to the present invention, and shows a stage where a resistmask is formed;

FIG. 4A shows the method for producing the transistor in an embodimentaccording to the present invention, and shows a stage of etching asecond conductive film and a third conductive film;

FIG. 4B shows the method for producing the transistor in an embodimentaccording to the present invention, and shows a stage of etching thethird conductive film;

FIG. 5A shows the method for producing the transistor in an embodimentaccording to the present invention, and shows a stage of forming anoxide semiconductor layer;

FIG. 5B shows the method for producing the transistor in an embodimentaccording to the present invention, and shows a stage of forming asecond insulating layer and a fourth conductive film;

FIG. 6 is a plan view showing a structure of a display device in anembodiment according to the present invention;

FIG. 7 shows an equivalent circuit of a pixel in the display device inan embodiment according to the present invention;

FIG. 8 is a plan view showing a structure of a pixel in the displaydevice in an embodiment according to the present invention;

FIG. 9A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 8;

FIG. 9B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 8;

FIG. 10A shows a structure of a display device and an influence ofcharges during an operation of the display device;

FIG. 10B shows a period in which a signal is applied to a gate electrodeduring the operation of the display device;

FIG. 11A shows a cross-sectional structure of a transistor in anembodiment according to the present invention;

FIG. 11B shows a cross-sectional structure of a transistor in anembodiment according to the present invention;

FIG. 12 is a plan view showing a method for producing the display devicein an embodiment according to the present invention;

FIG. 13A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 12;

FIG. 13B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 12;

FIG. 14A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 12;

FIG. 14B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 12;

FIG. 15 is a plan view showing the method for producing the displaydevice in an embodiment according to the present invention;

FIG. 16A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 15;

FIG. 16B is cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 15;

FIG. 17 is a plan view showing a method for producing the display devicein an embodiment according to the present invention;

FIG. 18A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 17;

FIG. 18B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 17;

FIG. 19 is a plan view showing the method for producing the displaydevice in an embodiment according to the present invention;

FIG. 20A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 19;

FIG. 20B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 19;

FIG. 21A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 19;

FIG. 21B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 19;

FIG. 22A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 19;

FIG. 22B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 19;

FIG. 23A is a cross-sectional view showing a method for producing adisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 19;

FIG. 23B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 19;

FIG. 24A is a cross-sectional view showing a method for producing adisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 19;

FIG. 24B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 19;

FIG. 25A is a cross-sectional view showing a method for producing adisplay device in an embodiment according to the present invention,taken along line A1-A2 in FIG. 19;

FIG. 25B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B1-B2 in FIG. 19;

FIG. 26 is a plan view showing a method for producing a display devicein an embodiment according to the present invention;

FIG. 27A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according the present invention, takenalong line A3-A4 in FIG. 26;

FIG. 27B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 26;

FIG. 28A is a cross-sectional view showing a method for producing adisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 26;

FIG. 28B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 26;

FIG. 29 is a plan view showing a method for producing the display devicein an embodiment according to the present invention;

FIG. 30A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 29;

FIG. 30B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 29;

FIG. 31A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 29;

FIG. 31B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 29;

FIG. 32A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 29;

FIG. 32B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 29;

FIG. 33A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 29;

FIG. 33B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 29;

FIG. 34 is a plan view showing the method for producing the displaydevice in an embodiment according to the present invention;

FIG. 35A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 34;

FIG. 35B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 34;

FIG. 36A is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 34;

FIG. 36B is a cross-sectional view showing the method for producing thedisplay device in an embodiment according to the present invention,taken along line B3-B4 in FIG. 34;

FIG. 37A is a cross-sectional view showing a method for producing adisplay device in an embodiment according to the present invention,taken along line A3-A4 in FIG. 34;

FIG. 37B is a cross-sectional view showing the method for producing thedisplay device in a embodiment according to the present invention, takenalong line B3-B4 in FIG. 34;

FIG. 38 is a plan view showing a structure of a pixel display device inan embodiment according to the present invention;

FIG. 39 is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A5-A6 in FIG. 38;

FIG. 40 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 41A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A7-A8 in FIG. 40;

FIG. 41B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B5-B6 in FIG. 40;

FIG. 42 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 43 is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A9-A10 in FIG. 42;

FIG. 44 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 45A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A11-A12 in FIG. 44;

FIG. 45B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B7-B8 in FIG. 44;

FIG. 46 is a plan view showing a method for producing a transistor in anembodiment according to the present invention, and shows a process ofdirecting lase light to decrease a resistance of the oxide semiconductorlayer;

FIG. 47A is a cross-sectional view showing a method for producing atransistor in an embodiment according to the present invention, andshows a process of directing the laser light from the second gateelectrode side;

FIG. 47B is a cross-sectional view showing a method for producing atransistor in an embodiment according to the present invention, andshows a process of directing the laser light from the first gateelectrode side;

FIG. 48A is a cross-sectional view showing a method for producing atransistor in an embodiment according to the present invention, andshows a process of directing the laser light from the second gateelectrode side;

FIG. 48B is a cross-sectional view showing a method for producing atransistor in an embodiment according to the present invention, andshows a process of directing the laser light from the first gateelectrode side;

FIG. 49 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 50A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A13-A14 in FIG. 49;

FIG. 50B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B9-B10 in FIG. 49;

FIG. 51 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 52A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A15-A16 in FIG. 51;

FIG. 52B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B11-B12 in FIG. 51;

FIG. 53A is a cross-sectional view showing a structure of a pixel in adisplay device in an embodiment according to the present invention,taken along line A15-A16 in FIG. 51;

FIG. 53B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B11-B12 in FIG. 51;

FIG. 54 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 55A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A17-A18 in FIG. 54;

FIG. 55B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B13-B14 in FIG. 54;

FIG. 56A is a cross-sectional view showing a structure of a pixel in adisplay device in an embodiment according to the present invention,taken along line A17-A18 in FIG. 54;

FIG. 56B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B13-B14 in FIG. 54;

FIG. 57 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 58A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A19-A20 in FIG. 57;

FIG. 58B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B15-B16 in FIG. 57;

FIG. 59 is a plan view showing a structure of a pixel in a displaydevice in an embodiment according to the present invention;

FIG. 60A is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line A21-A22 in FIG. 59;

FIG. 60B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B17-B18 in FIG. 59;

FIG. 61A is a cross-sectional view showing a structure of a pixel in adisplay device in an embodiment according to the present invention,taken along line A21-A22 in FIG. 59;

FIG. 61B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B17-B18 in FIG. 59;

FIG. 62A is a cross-sectional view showing a structure of a pixel in adisplay device in an embodiment according to the present invention,taken along line A19-A20 in FIG. 57; and

FIG. 62B is a cross-sectional view showing the structure of the pixel inthe display device in an embodiment according to the present invention,taken along line B15-B16 in FIG. 57.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings and the like. The present invention may becarried out in various embodiments, and should not be construed as beinglimited to any of the following embodiments. In the drawings, componentsmay be shown schematically regarding the width, thickness, shape and thelike, instead of being shown in accordance with the actual sizes, forthe sake of clear illustration. The drawings are merely examples and donot limit the present invention in any way. In the specification and thedrawings, components that are substantially the same as those describedor shown previously bear the identical reference signs thereto (or theidentical reference signs followed by letters “a”, “b” or the like), anddetailed descriptions thereof may be omitted. The terms “first”,“second” and the like used for elements are merely provided fordistinguishing the elements and do not have any other significanceunless otherwise specified.

In the specification and the claims, an expression that a component is“on” another component encompasses a case where such a component is incontact with the another component and also a case where such acomponent is above or below the another component, namely, a case wherestill another component is provided between such a component and theanother component, unless otherwise specified.

In order to increase the productivity of display devices, it is neededto shorten the time from when a transparent insulating substrate isprepared until an active matrix element substrate is completed. However,for producing an active matrix element substrate including acomplementary circuit that includes a transistor formed ofpolycrystalline silicon, eight or more photomasks are required.Polycrystalline silicon films produced by laser annealing are varied inthe crystallinity, and thus have a problem of deteriorating the displayquality when being used for a driving transistor.

In the meantime, a transistor formed of an oxide semiconductor requiresprecise control on the carrier concentration. An oxide semiconductor isa type of compound semiconductor containing a plurality of metal oxides,and thus requires, in a production process thereof, control on thecomposition, on the oxygen deficiency, and on impurities. In order tocontrol the carrier concentration of a channel by contriving the devicestructure, it is effective to provide a back gate. However, this hasproblems of complicating the structure and increasing the number ofphotomasks needed to produce the device.

In the case where the driving transistor is formed of oxidesemiconductor, a drain of the transistor is connected with a cathode ofthe organic EL element. Therefore, an electron transfer layer of theorganic EL element needs to be formed before a light emitting element;namely, the organic EL element needs to have a so-called inverted stackstructure. The organic EL element of an inverted stack structure has aproblem of not having characteristics equivalent to the characteristicsof an organic EL element having a normal stack structure, in which thelight emitting layer is stacked on a hole transfer layer. Some of theembodiments described below provide a display device solving one or aplurality of these problems.

Embodiment 1 1-1. Transistor Structure

FIG. 1 is a cross-sectional view showing a structure of a transistor 100a in embodiment 1 according to the present invention. The transistor 100a includes a first gate electrode 104, a first insulating layer 106, anoxide semiconductor layer 112, a second insulating layer 114, and asecond gate electrode 116, which are provided on a substrate 102 havingan insulating surface.

The first gate electrode 104 is located to face one of two main surfacesof the oxide semiconductor layer 112 (surface facing the substrate 102).The first insulating layer 106 is located between the oxidesemiconductor layer 112 and the first gate electrode 104. The secondgate electrode 116 is located to face the other main surface of theoxide semiconductor layer 112 (surface opposite to the surface facingthe substrate 102). The second insulating layer 114 is located betweenthe oxide semiconductor layer 112 and the second gate electrode 116. Thefirst gate electrode 104 and the second gate electrode 116 are locatedto partially overlap each other while having the first insulating layer106, the oxide semiconductor layer 112 and the second insulating layer114 between the first gate electrode 104 and the second gate electrode116. In the transistor 100 a, a channel is formed in a region where theoxide semiconductor layer 112 overlaps the first gate electrode 104 andthe second gate electrode 116. The first insulating layer 106 acts as agate insulating film in a region where the oxide semiconductor layer 112and the first gate electrode 104 overlap each other. The secondinsulating layer 114 acts as a gate insulating film in a region wherethe oxide semiconductor layer 112 and the second gate electrode 116overlap each other.

A first transparent conductive layer 108 a and a second transparentconductive layer 108 b are located between the oxide semiconductor layer112 and the first insulating layer 106. The first transparent conductivelayer 108 a and the second transparent conductive layer 108 b areprovided in contact with the oxide semiconductor layer 112. The firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b are located such that one end the first transparentconductive layer 108 a and one end of the second transparent conductivelayer 108 b overlap the first gate electrode 104 and the second gateelectrode 116. One of the first transparent conductive layer 108 a andthe second transparent conductive layer 108 b acts as a source region,and the other of the first transparent conductive layer 108 a and thesecond transparent conductive layer 108 b acts as a drain region. In thestructure shown in FIG. 1, the one ends of the first transparentconductive layer 108 a and the second transparent conductive layer 108 boverlap the first gate electrode 104 and the second gate electrode 116.Therefore, the oxide semiconductor layer 112 does not include any offsetregion (region having a high resistance). Thus, the level of on-currentis increased.

A first line 110 a is provided in contact with the first transparentconductive layer 108 a, and a second line 110 b is provided in contactwith the second transparent conductive layer 108 b. The first line 110 ais located between the first transparent conductive layer 108 a and theoxide semiconductor layer 112, and the second line 110 b is locatedbetween the second transparent conductive layer 108 b and the oxidesemiconductor layer 112. The first line 110 a and the second line 110 bare respectively located in contact with the first transparentconductive layer 108 a and the second transparent conductive layer 108b. This decreases the number of photolithography steps as describedbelow. In the transistor 100 a in this embodiment, A region where achannel is formed in the oxide semiconductor layer 112 is separated fromthe first line 110 a or the second line 110 b. Therefore, the oxidesemiconductor layer 112 is prevented from being contaminated with ametal material usable for the first line 110 a and the second line 110b.

1-2. Operations and Functions of the Transistor

In the transistor 100 a, the first gate electrode 104 is located to faceone of the two main surfaces of the oxide semiconductor layer 112, andthe second gate electrode 116 is located to face the other main surfaceof the oxide semiconductor layer 112. With such a structure, a constantpotential (fixed potential) may be applied to one of the first gateelectrode 104 and the second gate electrode 116, so that either one ofthe first gate electrode 104 and the second first gate electrode 116 actas a back gate. The transistor 100 a is substantially of an n-channeltype. Therefore, for example, one of the first gate electrode 104 andthe second gate electrode 116 may be supplied with a potential lowerthan a source potential, so that the gate electrode supplied with such apotential acts as a back gate electrode. Thus, a threshold voltage ofthe transistor 100 a is controlled. The first gate electrode 104 and thesecond gate electrode 116 of the transistor 100 a may be supplied withthe same gate voltage, so that the transistor 100 a acts as a dual gatetransistor. Thus, the transistor 100 a increases the level of on-currentand improves the frequency characteristics.

1-3. Oxide Semiconductor Layer

The oxide semiconductor layer 112 contains one or a plurality ofelements selected from indium (In), zinc (Zn), gallium (Ga), tin (Sn),aluminum (Al), and magnesium (Mg). For example, an oxide semiconductormaterial used to form the oxide semiconductor layer 112 may be afour-component oxide material, a three-component oxide material, atwo-component oxide material or a one-component oxide material showingsemiconductor characteristics. Examples of the four-component oxidematerial include an In₂O₃—Ga₂O₃—SnO₂—ZnO-based oxide material and thelike. Examples of the three-component oxide material include anIn₂O₃—Ga₂O₃—ZnO-based oxide material, an In₂O₃—SnO₂—ZnO-based oxidematerial, an In₂O₃—Al₂O₃—ZnO-based oxide material, aGa₂O₃—SnO₂—ZnO-based oxide material, a Ga₂O₃—Al₂O₃—ZnO-based oxidematerial, an SnO₂—Al₂O₃—ZnO-based oxide material, and the like. Examplesof the two-component oxide material include an In₂O₃—ZnO-based oxidematerial, an SnO₂—ZnO-based oxide material, an Al₂O₃—ZnO-based oxidematerial, an MgO—ZnO-based oxide material, an SnO₂—MgO-based oxidematerial, an In₂O₃—MgO-based oxide material, and the like. Examples ofthe one-component oxide material include an In₂O₃-based metal oxidematerial, an SnO₂-based metal oxide material, a ZnO-based metal oxidematerial, and the like. The above-listed oxide semiconductors mayinclude silicon (Si), nickel (Ni), tungsten (W), hafnium (Hf), ortitanium (Ti). For example, the In—Ga—Zn—O-based oxide material is anoxide material containing at least In, Ga and Zn. There is no specificlimitation on the composition ratio thereof. In other words, the oxidesemiconductor layer 112 may be formed of a thin film represented bychemical formula InMO₃(ZnO)_(m) (m>0). M represents one or a pluralityof metal elements selected from Ga, Al, Mg, Ti, Ta, W, Hf and Si. Theoxide material contained in each of the four-component oxide materials,the three-component oxide materials, the two-component oxide materialsand one-component oxide materials listed above is not limited to havinga stoichiometric composition, but may have a composition shifted fromthe stoichiometric composition.

The oxide semiconductor layer 112 is formed by sputtering. For example,the oxide semiconductor layer 112 may be formed by use of a sputteringtarget compatible to any of the four-component oxide materials, thethree-component oxide materials, the two-component oxide materials andthe one-component oxide materials listed above and also by use of, assputtering gas, noble gas such as argon (Ar), xenon (Xe) or the like ormixed gas of noble gas and oxygen (O₂).

The oxide semiconductor layer 112 desirably has a carrier concentrationof about 1×10¹⁵/cm³ to 5×10¹⁸/cm³ in order to form a channel layer ofthe transistor 100 a. As long as the carrier concentration of the oxidesemiconductor layer 112 is in this range, a normally-off transistor isformed. In addition, an on-current/off-current ratio (on/off ratio) ofabout 10⁷ to 10¹⁰ is provided.

1-4. Transparent Conductive Layers

The first transparent conductive layer 108 a and the second transparentconductive layer 108 b are formed of a metal oxide material, a metalnitride material, or a metal oxide nitride material, all of which areconductive. Examples of the metal nitride material usable for the firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b include indium tin oxide (In₂O₃.SnO₂: ITO), indium zincoxide (In₂O₃.ZnO: IZO), and tin oxide (SnO₂). Such a metal oxidematerial forms a good ohmic contact with the oxide semiconductor layer112.

Examples of the metal oxide material usable for the first transparentconductive layer 108 a and the second transparent conductive layer 108 balso include titanium oxide (TiO_(x)) and the like. Examples of themetal nitride material usable for the first transparent conductive layer108 a and the second transparent conductive layer 108 b include titaniumnitride (TiN_(x)), zirconium nitride (ZrN_(x)), and the like. Examplesof the metal oxide nitride material usable for the first transparentconductive layer 108 a and the second transparent conductive layer 108 binclude titanium oxide nitride (TiO_(x)N_(y)), tantalum oxide nitride(TaO_(x)N_(y)), zirconium oxide nitride (ZrO_(x)N_(y)), hafnium oxidenitride (HfO_(x)N_(y)), and the like. The metal oxide materials, themetal nitride materials, and the metal oxide nitride materials describedabove may contain trace amount of metal element in order to improve theconductivity. For example, titanium oxide doped with niobium(TiO_(x):Nb) may be used. Use of such a metal oxide material, such ametal nitride material, or such a metal oxide nitride materialguarantees stability even in the case where the first transparentconductive layer 108 a and the second transparent conductive layer 108 bare in contact with the first line 110 a and the second line 110 b,respectively. Namely use of such a metal oxide material, such a metalnitride material, or such a metal oxide nitride material prevents anoxidation-reduction reaction (local cell reaction) with aluminum (Al)having a lower potential.

1-5. Insulating Layers

The first insulating layer 106 and the second insulating layer 114 areformed of an inorganic insulating material. Examples of the inorganicinsulating material usable for the first insulating layer 106 and thesecond insulating layer 114 include silicon oxide, silicon nitride,silicon oxide nitride, aluminum oxide, and the like. The firstinsulating layer 106 and the second insulating layer 114 each have asingle-layer structure, or a stack structure including a plurality offilms, formed of such an organic insulating material. For example, thefirst insulating layer 106 may include a silicon nitride film and asilicon oxide film stacked in this order from the substrate 102 side.The second insulating layer 114 may include a silicon oxide film and asilicon nitride film stacked in this order from the oxide semiconductorlayer 112 side. The first insulating layer 106 and the second insulatinglayer 114, in the case of including a plurality of organic insulatingfilms, alleviate the action of an internal stress and also improve thebarrier property against water vapor or the like.

It is preferable that surfaces of the first insulating layer 106 and thesecond insulating layer 114 that are in contact with the oxidesemiconductor layer 112 are formed of a silicon oxide film, a siliconoxide nitride or an aluminum oxide film. Since such an oxide insulatingfilm in contact with the oxide semiconductor layer 112 (in other words,a nitride insulating film is not in contact with the oxide semiconductorlayer 112), diffusion of impurities such as hydrogen or the like, whichgenerates a donor in the oxide semiconductor layer 112, is suppressed.Since the oxide insulating film is provided in contact with the oxidesemiconductor layer 112, a defect (donor) caused by oxygen deficiency isprevented from being caused to the oxide semiconductor layer 112.

1-6. Gate Electrodes

The first gate electrode 104 and the second gate electrode 116 areformed of a metal material such as aluminum (Al), molybdenum (Mo),tungsten (W), zirconium (Zr) or the like. For example, the first gateelectrode 104 and the second gate electrode 116 may each be formed of afilm of aluminum (Al), a molybdenum-tungsten alloy (MoW), or the like.The first gate electrode 104 and the second gate electrode 116 may beformed of an aluminum alloy, a copper alloy, or a silver alloy. Examplesof the aluminum alloy usable for the first gate electrode 104 and thesecond gate electrode 116 include an aluminum-neodymium alloy (Al—Nd),an aluminum-neodymium-nickel alloy (Al—Nd—Ni), an aluminum-carbon-nickelalloy (Al—C—Ni), a copper-nickel alloy (Cu—Ni), and the like.Alternatively, the first gate electrode 104 and the second gateelectrode 116 may each be formed of a transparent conductive film ofindium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO) or thelike.

1-7. Lines

The first line 110 a and the second line 110 b are formed of a metalmaterial having a high conductivity such as aluminum (Al), copper (Cu)or the like. For example, the first line 110 a and the second line 110 bare formed of an aluminum alloy, a copper alloy, or a silver alloy.Examples of the aluminum alloy usable for the first line 110 a and thesecond line 110 b include an aluminum-neodymium alloy (Al—Nd), analuminum-titanium alloy (Al—Ti), an aluminum-silicon alloy (Al—Si), analuminum-neodymium-nickel alloy (Al—Nd—Ni), an aluminum-carbon-nickelalloy (Al—C—Ni), a copper-nickel alloy (Cu—Ni), and the like. Use ofsuch a metal material provides heat resistance and decreases the lineresistance.

1-8. Production Method

Now, a method for producing the transistor 100 a will be described. FIG.2A shows a stage of forming the first gate electrode 104 on thesubstrate 102. The substrate 102 may be, for example, a transparentinsulating substrate. The transparent insulating substrate is formed ofnon-alkali glass such as aluminosilicate glass, aluminoborosilicateglass or the like, or quartz.

First, a first conductive film 103 is formed on one surface of thesubstrate 102. Then, a resist mask is formed on the first conductivefilm 103 by a photolithography step, and the first gate electrode 104formed by an etching step. The first conductive film 103 is not limitedto having any specific thickness, but is formed to have a thickness ofabout 100 nm to 2000 nm. It is preferable that the first gate electrode104 has tapered ends as seen in a cross-sectional view. The first gateelectrode 104 has tapered ends and thus is covered with the firstinsulating layer 106 with certainty. Therefore, in the etching step offorming the first gate electrode 104, it is preferable to performanisotropic etching on the first conductive film 103 while chemicallymilling the resist mask; namely, to perform so-called taper etching. Theresist mask that is left after the formation of the first gate electrode104 is removed by use of a releasing solution or by an ashing process.

FIG. 2B shows stage of forming the first insulating layer 106, a secondconductive film 107, and a third conductive film 109 on the first gateelectrode 104. From the second conductive film 107, the firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b are formed. From the third conductive film 109, the firstline 110 a and the second line 110 b are formed. The first insulatinglayer 106 is formed of an inorganic insulating film. For example, thefirst insulating layer 106 may be a film formed of one or a plurality ofmaterials selected from silicon oxide, silicon nitride, and siliconoxide nitride. In this case, the first insulating layer 106 is formed byplasma CVD (Chemical Vapor Deposition). Alternatively, the firstinsulating layer 106 may be an aluminum oxide film. In this case, thefirst insulating layer 106 is formed by sputtering by use of an aluminasputtering target. The first insulating layer 106 is used as a gateinsulating layer. Therefore, the first insulating layer 106 is formed tohave a thickness of about 100 nm to 500 nm.

The second conductive film 107 used to form the first transparentconductive layer 108 a and the second transparent conductive layer 108 bis formed of a metal oxide material, a metal nitride material or a metaloxide nitride material, all of which are conductive. The secondconductive film 107 is formed by sputtering. The second conductive film107 used to form the first transparent conductive layer 108 a and thesecond transparent conductive layer 108 b is formed of, for example, aconductive metal oxide material to have a thickness of 30 nm to 200 nm.The third conductive film 109 used to form the first line 110 a and thesecond line 110 b is formed of a metal material or an alloy material bysputtering. The third conductive film 109 used to form the first line110 a and the second line 110 b is formed of a metal material to have athickness of 200 nm to 2000 nm in order to have a low resistance.

FIG. 3A shows a lithography step of forming the first line 110 a, thesecond line 110 b, the first transparent conductive layer 108 a and thesecond transparent conductive layer 108 b. In this example, amulti-gradation exposure method (halftone exposure method) is used.Specifically, the patterns of the first line 110 a, the second line 110b, the first transparent conductive layer 108 a and the secondtransparent conductive layer 108 b are formed by one photomask.

A positive photoresist film 205 is formed on the third conductive film109. A multi-gradation photomask 201 is used for exposing thephotoresist mask 205 to light. A multi-gradation photomask is availablein two types: a gray tone photomask, which has a multi-gradation scalepattern (like a pattern 203) having slits of a resolution equal to, orlower than, the resolution of an exposure device and realizesmulti-gradation exposure by the slits blocking a part of light; and ahalftone photomask, which realizes multi-gradation exposure by use of asemi-transmissive film. In this embodiment, both types ofmulti-gradation photomask are usable. As a result of using themulti-gradation photomask 201 for the exposure, the photoresist film 205has three portions formed therein, namely, an exposed portion, agradation-exposed portion, and a non-exposed portion.

Then, the photoresist film 205 is developed to a resist mask 207 aincluding regions having different thicknesses as shown in FIG. 3B. Asshown in FIG. 3B, the resist mask 207 a is thicker in a regioncorresponding to regions of the third conductive film 109 where thefirst line 110 a and the second line 110 b are to be formed, and isthinner in the remaining region.

The third conductive film 109 and the second conductive film 107 areetched by use of the resist mask 207 a. There is no specific limitationon the conditions for the etching. For example, the third conductivefilm 109, which is formed of a metal material, is wet-etched by use of amixed acid etchant, and the second conductive film 107, which is formedof a metal oxide material, is dry-etched by use of chlorine-based gas.On this stage, the first transparent conductive layer 108 a and thesecond transparent conductive layer 108 b are formed. After the etching,an ashing process is performed to remove the thinner region of theresist mask 207 a to expose a surface of the third conductive film 109.FIG. 4A shows a resist mask 207 b after the ashing process. The resistmask 207 b is left on the third conductive film 109.

Next, etching is performed on the exposed third conductive film 109.This etching is wet-etching performed by use of, for example, a mixedacid etchant. The second conductive film 107, which is formed of a metaloxide material or the like, is not easily etched away by the mixed acidetchant, and thus the selection ratio is relatively high. Therefore, theshape of the first transparent conductive layer 108 a and the secondtransparent conductive layer 108 b below the third conductive film 109is kept unchanged. FIG. 4B shows a stage where the first line 110 a andthe second line 110 b are formed as a result of the etching performed onthe third conductive film 109. After the third conductive film 109 isetched, the resist mask 207 b is removed by ashing.

Surfaces of the first transparent conductive layer 108 a and the secondtransparent conductive layer 108 b already formed are exposed to oxygenplasma by the ashing process. However, titanium (Ti), tantalum (Ta),hafnium (Hf) or zirconium (Zr) contained as a component of the firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b is conductive even when being oxidized. Therefore, eventhough being exposed to oxygen plasma, the surfaces of the firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b form a good contact with the oxide semiconductor layer 112formed in a later step.

FIG. 5A show a stage of forming the oxide semiconductor layer 112. Theoxide semiconductor layer 112 is formed to cover the first transparentconductive layer 108 a, the second transparent conductive layer 108 b,the first line 110 a, and the second line 110 b. The oxide semiconductorlayer 112 is formed by sputtering. As a sputtering target, a sinteredoxide semiconductor material is used. The oxide semiconductor layer 112is formed to have a thickness of 20 nm to 100 nm, for example, 30 nm to50 nm.

FIG. 5B shows a stage of forming the second insulating layer 114 and afourth conductive film 115 on the oxide semiconductor layer 112. Thesecond insulating layer 114 formed in substantially the same manner asthe first insulating layer 106. The fourth conductive film 115 is formedin substantially the same manner as the first conductive film 103. Then,the fourth conductive film 115 is etched to form the second gateelectrode 116. Thus, the transistor 110 a shown in FIG. 1 is produced.

According to the method for producing the transistor 100 a in thisembodiment, a multi-gradation photomask is used to decrease the numberof photomasks required to produce the transistor 100 a. The use of themulti-gradation photomask allows a plurality patterns (the firsttransparent conductive layer 108 a, the second transparent conductivelayer 108 b, the first line 110 a and the second line 11 b) to be formedby performing exposure merely once. This increases the productivity ofintegrated circuit elements each including the transistor 100 a and alsodecreases the production cost.

As shown in FIG. 1, neither the first line 110 a nor the second line 110b overlaps the first gate electrode 104 or the second gate electrode116. The first line 110 a and the second line 110 b as far as possiblefrom the channel region of the transistor 100 a (region where the firstgate electrode 104 and the second gate electrode 116 overlap the oxidesemiconductor layer 112), so that the channel region is prevented frombeing contaminated with a metal element. For example, copper (Cu), whichmay be used as a material of the first line 110 a and the second line110 b, is a killer impurity to the oxide semiconductor, which is ann-type semiconductor (impurity that deteriorates the characteristics ofthe oxide semiconductor and destroys the oxide semiconductor). In thisembodiment, the first line 110 a and the second line 110 b are locatedas far as possible from the channel region of the transistor 110 a.Therefore, even if the first line 110 a and the second line 110 bcontain copper (Cu), the oxide semiconductor layer 112 is suppressedfrom being contaminated with copper (Cu).

Embodiment 2

In embodiment 2, an example of display device 120 including a transistorhaving substantially the same structure as that of the transistordescribed in embodiment 1 will be described. As shown in FIG. 6, thedisplay device 120 includes a display region 121 including a pluralityof pixels 122, a scanning line driving circuit 123, and a data linedriving circuit 125. Although not shown in FIG. 6, the plurality ofpixels 122 each include an organic EL element acting as a displayelement and a transistor driving the organic EL element.

2-1. Equivalent Circuit

FIG. 7 is an equivalent circuit diagram of each of the pixels 122included in the display device 120 in this embodiment. The pixel 122includes a selection transistor 124, a driving transistor 126, acapacitance element 128 and an organic EL element 130. The selectiontransistor 124 and the driving transistor 126 each have substantiallythe same structure as that of the transistor 100 a described inembodiment 1. Namely, FIG. 7 shows the transistors of a dual gatestructure. The selection transistor 124 includes a first gate electrode104 b and a second gate electrode 116 b, and the driving transistor 126includes a first gate electrode 104 a and a second gate electrode 116 a.

In FIG. 7, the selection transistor 124 and the driving transistor 126are each of an n-channel type. Gates of the selection transistor 124(the first gate electrode 104 b and the second gate electrode 116 b) areconnected with a gate signal line 132 a. One of input/output terminals(source and drain) of the selection transistor 124 is connected with adata signal line 134, and the other of the input/output terminals isconnected with gates of the driving transistor 126 (the first gateelectrode 104 a and the second gate electrode 116 a). The gates of thedriving transistor 126 (the first gate electrode 104 a and the secondgate electrode 116 a) are connected with the other of the input/outputterminals of the selection transistor 124. A drain of the drivingtransistor 126 is connected with the organic EL element 130, and asource of the driving transistor 126 is connected with a second commonline 136 b. One of two terminals of the capacitance element 128 isconnected with the other of the input/output terminals (source anddrain) of the selection, transistor 124. The other of the two terminalsof the capacitance element 128 is connected with a first common line 136a. The first common line 136 a and the second comma line 136 b aresupplied with, for example, ground potential.

One of two terminals of the organic EL element 130 is connected with thedrain of the driving transistor 126, and the other of the two terminalsof the organic EL element 130 is connected with a power supply line 138.The power supply line 138 is supplied with a power supply potential VDD,which is higher than the potential of each of the common lines 136 a and136 b. In this embodiment, the terminal of the organic EL element 130that is connected with the drain of the driving transistor 126 is acathode electrode, and the terminal of the organic EL element 130 thatis connected with the power supply line 138 is an anode electrode.

2-2. Pixel Structure

FIG. 8 shows an example of planar structure of a pixel 122 acorresponding to the equivalent circuit shown in FIG. 7. FIG. 9A shows across-sectional structure taken along line A1-A2 in FIG. 8, and FIG. 9Bshows a cross-sectional structure taken along line B1-AB in FIG. 8. FIG.9A shows a cross-sectional structure of the driving transistor 126 andthe organic EL element 130. FIG. 9B shows a cross-sectional structure ofthe selection transistor 124 and the capacitance element 128. Thefollowing description will be made with reference to FIG. 8, FIG. 9A andFIG. 9B. In the plan view of the pixel 122 a shown in FIG. 8A, thestructure of the organic EL element 130 is omitted.

2-2-1. Driving Transistor

The driving transistor 126 has substantially the same structure as thatof the transistor 100 a described in embodiment 1. Specifically, thedriving transistor 126 includes the first gate electrode 104 a, thefirst insulating layer 106, a first oxide semiconductor layer 112 a, thesecond insulating layer 114, and the second gate electrode 116 a, whichare stacked. The first gate electrode 104 a is provided between thesubstrate 102 and the first insulating layer 106. The second gateelectrode 116 a is provided on the second insulating layer 114 (on thesurface of the second insulating layer 114 opposite to the surfacethereof facing the substrate 102).

The first transparent conductive layer 108 a and the second transparentconductive layer 108 b are provided between the first insulating layer106 and the first oxide semiconductor layer 112 a. The first transparentconductive layer 108 a and the second transparent conductive layer 108 bare provided to hold the first gate electrode 104 a and the second gateelectrode 116 a therebetween as seen in a plan view. The firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b are provided in contact with the first oxide semiconductorlayer 112 a.

The first transparent conductive layer 108 a of the driving transistor126, or a region of the driving transistor 126 where the firsttransparent conductive layer 108 a is in contact with the first oxidesemiconductor layer 112 a, is a drain region. The second transparentconductive layer 108 b of the driving transistor 126, or a region of thedriving transistor 126 where the second oxide semiconductor layer 112 bis in contact with the second transparent conductive layer 108 b, is asource region.

The second transparent conductive layer 108 b of the driving transistor126 is electrically connected with the first oxide semiconductor layer112 a, the first common line 136 a and the second common line 136 b. Thefirst common line 136 a is provided in the same layer structure as thatof the first gate electrode 104 a, and the second common line 136 b isprovided in the same layer structure as that of the data signal line 134which electrically connected to the selection transistor 124. The firstcommon line 136 a and the second transparent conductive layer 108 b areelectrically connected with each other via a first contact hole 117 aformed in the first insulating layer 106. The second common line 136 bis in direct contact with a top surface of the second transparentconductive layer 108 b.

The first insulating layer 106 includes, for example, a first siliconnitride film 141 a and a first silicon oxide film 140 a stacked in thisorder from the substrate 102 side. The second insulating layer 114includes a second silicon oxide film 140 b and a second silicon nitridefilm 141 b stacked in this order from the first oxide semiconductorlayer 112 a side.

A channel of the driving transistor 126 is formed in a region where thefirst oxide semiconductor layer 112 a overlaps the first gate electrode104 a and the second gate electrode 116 a. Therefore, the first oxidesemiconductor layer 112 a is provided in contact with the silicon oxidefilms 140 a and 140 b in the region where the channel is formed. Thefirst oxide semiconductor layer 112 a is provided in contact with theinsulating oxide films, and thus generation of oxygen deficiency in thefirst oxide semiconductor layer 112 a is suppressed. It is desirablethat the silicon oxide films 140 a and 140 b do not have oxygendeficiency so as not to draw out oxygen from the first oxidesemiconductor layer 112 a. Rather, it is even preferable that thesilicon oxide films 140 a and 140 b contain an excessive amount ofoxygen. A reason for this is that the silicon oxide films 140 a and 140b, in the case of containing an excessive amount of oxygen, may be asource of oxygen for the first oxide semiconductor layer 112 a. The“silicon oxide film containing an excessive amount of oxygen”encompasses a silicon oxide film containing an excessive amount ofoxygen with respect to the chemical stoichiometric composition, and alsoencompasses a silicon oxide film containing an excessive amount ofoxygen in a lattice thereof. The first insulating layer 106 and thesecond insulating layer 114 may be formed of silicon oxide nitride oraluminum oxide instead of silicon oxide.

The driving transistor 126 is covered with a fattening layer 142. Theflattening layer 142 is formed of an organic resin material such as, forexample, an acrylic resin, a polyimide resin, an epoxy resin, apolyamide resin or the like. The flattening layer 142 is formed asfollows. A composition containing a precursor of an organic resinmaterial is applied, and a surface of the film formed by the compositionis flattened by a levelling action of the film. In another embodiment,the flattening layer 142 may be formed as follows. An inorganicinsulating film such as a silicon oxide film or the like is formed byplasma CVD or the like, and then a surface of the inorganic insulatingfilm is flattened by chemical mechanical polishing (CMP).

An opening 144 is provided in the flattening layer 142 and the secondinsulating layer 114. A first electrode 146, which is the cathodeelectrode of the organic EL element 130, is located to overlap theopening 144. The organic EL element 130 is formed of a plurality oflayers stacked in the region of the opening 144.

In this embodiment, the driving transistor 126 has a dual gatestructure, and thus improves the current driving capability thereof.Therefore, the driving transistor 126 provides a sufficient level ofcurrent to drive the organic EL element 130. Even if the operating pointof the organic EL element 130 is changed, the driving transistor 126provides constant current driving in accordance with the change in theoperating point.

2-2-2. Selection Transistor

The selection transistor 124 has substantially the same structure asthat of the transistor 100 a described in embodiment 1. Specifically,the selection transistor 124 includes the first gate electrode 104 b,the first insulating layer 106, a second oxide semiconductor layer 112b, the second insulating layer 114, and the second gate electrode 116 b,which are stacked. A channel of the second transistor 124 is formed in aregion where the second oxide semiconductor layer 112 b overlaps thefirst gate electrode 104 b and the second gate electrode 116 b. A thirdtransparent conductive layer 108 c and a fourth transparent conductivelayer 108 d are provided between the first insulating layer 106 and thesecond oxide semiconductor layer 112 b. The third transparent conductivelayer 108 c and the fourth transparent conductive layer 108 d areprovided in contact with the second oxide semiconductor layer 112 b, andthus act as a source region and a drain region. The third transparentconductive layer 108 c and the fourth transparent conductive layer 108 dare provided to hold the first gate electrode 104 b and the second gateelectrode 116 b therebetween as seen in a plan view.

The third transparent conductive layer 108 c is electrically connectedwith the data signal line 134. The data signal line 134 is provided inthe same layer structure as that of the line layer 110 provided betweenthe transparent conductive layer 108 and the oxide semiconductor layer112 described in the embodiment 1. The data signal line 134 is in directcontact with a top surface of the third transparent conductive layer 108c. The second oxide semiconductor layer 112 b extends to a region wherethe data signal line 134 is located and covers the data signal line 134.The data signal line 134 is in direct contact with the third transparentconductive layer 108 c and thus has a larger contact area size than inthe case where the data signal line 134 is connected with the thirdtransparent conductive layer 108 c via a contact hole. Therefore, thecontact resistance is decreased. A top surface and side surfaces of thedata signal line 134 are covered with the second oxide semiconductorlayer 112 b, and thus the data signal line 134 is not exposed to anoxidizing atmosphere or a reducing atmosphere during the production ofthe display device 120. Therefore the data signal line 134 suppressesthe resistance surface thereof from increasing.

2-2-3. Capacitance Element

The capacitance element 128 includes a first capacitance electrode 160a, the first insulating layer 106, the fourth transparent conductivelayer 108 d, and a second capacitance electrode 160 b. The firstcapacitance electrode 160 a is provided in the same layer structure asthat of the first gate electrode 104, and the second capacitanceelectrode 160 b is provided in the same layer structure as that of thedata signal line 134. The fourth transparent conductive layer 108 d iselectrically connected with the second capacitance electrode 160 b, andthus substantially acts as the electrode of the capacitance element 128.

The second oxide semiconductor layer 112 b and the second insulatinglayer 114 are provided on the second capacitance electrode 160 b. Thesecond capacitance electrode 160 b is electrically connected with thesecond gate electrode 116 a via a second contact hole 117 b runningthrough the second insulating layer 114 and the second oxidesemiconductor layer 112 b.

2-2-4. Organic EL Element

The organic EL element 130 includes the first electrode 146corresponding to the cathode electrode, an electron transfer layer 148,an electron injection layer 150, a light emitting layer 152, a holetransfer layer 154, a hole injection layer 156, and a second electrode158 corresponding to the anode electrode, which are stacked from thesubstrate 102 side. A structure of an organic EL element in which a holetransfer layer, a light emitting layer, an electron transfer layer, anda cathode electrode are stacked in this order from the side of the anodeelectrode close to the substrate is referred to as a “normal stackstructure”. In the organic EL element 130 in this embodiment, theelectron transfer layer 148, the light emitting layer 152, the holetransfer layer 154 and the like are stacked in this order from the sideof the cathode electrode close to the substrate 102. This structure isreferred to as an “inverted stack structure”. In this embodiment, thedriving transistor 126 is of an n-channel type. Therefore, if theorganic EL element has a normal stack structure, the source is connectedwith the anode electrode. In this case, there is a problem that thelevel of drain current of the driving transistor is changed inaccordance with the change in the characteristics of the organic ELelement. However, in the case where the organic EL element 130 has aninverted stack structure as in this embodiment, the drain of then-channel type driving transistor 126 is connected with the cathode ofthe organic EL element 130. Therefore, a circuit configuration in whichthe drain current is not much influenced by the change in thecharacteristics of the organic EL element 130 is provided.

On a top surface of the flattening layer 142 and in the opening 144provided in the flattening layer 142 and the second insulating layer114, the electron transfer layer 148, the electron injection layer 150,the light emitting layer 152, the hole transfer layer 154, the holeinjection layer 156, and the second electrode 158 acting as the anodeelectrode are stacked. A region where a stacked body including theseelements overlaps the first electrode 146 corresponding to the cathodeelectrode is a light emitting region of the organic EL element 130.

The organic EL element 120 in this embodiment is of a so-called bottomemission type, which outputs light toward the substrate 102.Hereinafter, each of the layers included in the organic EL element 130will be described in detail.

2-2-4-1. Cathode Electrode

As a material of a cathode electrode of an organic EL element, analuminum-lithium alloy (AlLi), a magnesium-silver alloy (MgAg) or thelike is conventionally used. However, these materials are easilydeteriorated by the influence of oxygen or moisture in the air, and thusare difficult to handle. These materials for the cathode electrode aremetal materials, and thus are not suitable to an organic EL element thathas an inverted stack structure and is of a bottom emission type.

In the organic EL element 130 this embodiment, the first electrode 146,which is a cathode electrode, is formed of a transparent conductivematerial, and thus a bottom emission type structure is realized for theorganic EL element 130. Specifically, the first transparent conductivelayer 108 a of the driving transistor 126 extends to the region of theorganic EL element 130 to act as the first electrode 146, which is acathode electrode. With such an arrangement, the driving transistor 126and the organic EL element 130 are electrically connected with eachother with a simple structure. For example, in the case where aninterlayer insulating layer is provided between the driving transistorand the organic EL element, a contact hole needs to be provided toconnect the driving transistor and the organic EL element. By contrast,the structure of the first pixel 122 a in this embodiment does notrequire a contact hole.

The first electrode 146 as the cathode electrode is formed of the sameconductive film as that of the first transparent conductive layer 108 a.The first transparent conductive layer 108 a is formed, of a metal oxidematerial, a metal nitride material, or a metal oxide nitride material,all of which are conductive. A conductive film formed of such a materialhas a bandgap of 2.8 eV or greater, preferably of 3.0 eV or greater, andtherefore transmits almost all of light of a visible region. Therefore,such a material is usable for an electrode on the light output side ofthe organic EL element 130.

On the first electrode 146 corresponding to the cathode electrode, thefirst oxide semiconductor layer 112 a extending from the drivingtransistor 126 may be provided. The oxide semiconductor layer 112 a hasa bandgap of 3 eV or greater, and thus is visible light-transmissive. Asdescribed below, in this embodiment, the electron transfer layer 148 isformed of a metal oxide. The first oxide semiconductor layer 112 aformed of the same material, or the same type of material, as theelectron transfer layer 148 is located between the electron transferlayer 148 and the first electrode 146 corresponding to the cathodeelectrode, so that formation of an electron injection barrier isprevented. In other words, the first oxide semiconductor layer 112 aextending from the channel region of the driving transistor 126 may beused as a part of the electron transfer layer 148 in contact with thefirst electrode 146 corresponding to the cathode electrode.

2-2-4-2. Electron Transfer Layer

The electron transfer layer 148 is formed of a metal oxide material.Examples of the metal oxide material usable for the electron transferlayer 148 include substantially the same materials described inembodiment 1, specifically, a four-component oxide material, athree-component oxide material, a two-component oxide material, and aone-component oxide material. These metal oxide materials may be in anamorphous state, a crystalline state or a mixed phase of an amorphousstate and a crystalline state. The electron transfer layer 148 is formedof, for example, one or a plurality of materials selected from an oxideof indium, an oxide of zinc, an oxide of gallium (Ga), and an oxide oftin (Sn). Such a metal oxide material should not absorb visible lightand needs to be transparent, and thus is required to have a bandgap of3.0 eV or greater. The electron transfer layer 148 may have a maximumpossible thickness to prevent short circuiting between the anodeelectrode and the cathode electrode. The electron transfer layer 148 maybe formed by sputtering, vacuum vapor deposition, application or thelike. The electron transfer layer 148 is formed by such a method to havea thickness of 50 nm to 1000 nm.

The electron transfer layer 148 has a carrier concentration of 1/10 orless, preferably 1/100 or less, of that of the oxide semiconductor layer112 a. In other words, the carrier concentration of the region where theoxide semiconductor layer 112 a is in contact with the electron transferlayer 148 is at least 10 times, preferably 100 times, the carrierconcentration of the electron transfer layer 148. Specifically, thecarrier concentration of the electron transfer layer 148 is 10¹³ to10¹⁷/cm³, whereas the carrier concentration of the oxide semiconductorlayer 112 a is 10¹⁵ to 10¹⁹/cm³. The difference between the carrierconcentrations of the electron transfer layer 148 and the oxidesemiconductor layer 112 a is at least one digit, preferably at least twodigits. The oxide semiconductor layer 112 a has a carrier concentrationof is 10¹⁵ to 10¹⁹/cm³, and thus decreases the resistance loss in theelectric connection between the driving transistor 126 and the organicEL element 130, and suppresses the driving voltage from increasing. Inthe case where the carrier concentration of the electron transfer layer148 is 10²⁶/cm³ or greater, the excited state in the light emittinglayer 152 is deactivated and the light emission efficiency is decreased.By contrast, in the case where the carrier concentration of the electrontransfer layer 148 is less than 10¹³/cm³, the number of carrierssupplied to the light emitting layer 152 is decreased and thus asufficient level of luminance is not provided. As described above, theoxide semiconductor layer 112 a extending from the driving transistor126 is provided in contact with the electron transfer layer 148, and thecarrier concentrations of the oxide semiconductor layer 112 a and theelectron transfer layer 148 are made different from each other, so thatthe driving voltage is prevented from increasing and the light emissionefficiency of the organic EL element 130 is improved.

2-2-4-3. Electron Injection Layer

In an organic EL element, an electron injection layer is used in orderto decrease the energy barrier and thus to inject the electrons from thecathode electrode into an electron transfer material. In thisembodiment, the electron injection layer 150 is used in order to allowthe electrons to be injected easily from the electron transfer layer 148formed of an oxide semiconductor into the light emitting layer 152.Thus, the electron injection layer 150 is provided between the electrontransfer layer 148 and the light emitting layer 152.

It is desirable that the electron injection layer 150 is formed of amaterial having a small work function in order to allow the electrons tobe injected easily into the light emitting layer 150 formed of anorganic material. The electron injection layer 150 is formed of an oxideof calcium (Ca) and an oxide of aluminum (Al). The electron injectionlayer 150 is preferably formed of, for example, C12A7 (12CaO.7Al₂O₃)electride. C12A7 (12CaO.7Al₂O₃) electride has semiconductorcharacteristics, is controllable to have a desired level of resistancefrom a high resistance to a low resistance, and has a work function of2.4 eV to 3.2 eV, which is about the same as that of alkaline metal. Forthese reasons, C12A7 (12CaO.7Al₂O₃) electride is preferably usable forthe electron injection layer 150.

The electron injection layer 150 of C12A7 electride is formed bysputtering by use of a polycrystal of C12A7 electride as a target. C12A7electride has semiconductor characteristics, and thus the electroninjection layer 150 may be formed to have a thickness of 1 nm to 100 nm.Regarding C12A7 electride, it is preferable that the molar ratio ofCa:Al is in the range of 13:13 to 11:16. C12A7 electride is formed bysputtering and thus is preferably amorphous. Alternatively, C12A7electride may be crystalline.

C12A7 electride is stable in the atmosphere, and thus has an advantageof being easier to handle than an alkaline metal compound conventionallyused for an electron injection layer such as lithium fluoride (LiF),lithium oxide (Li₂O), sodium chloride (NaCl), potassium chloride(KCl),or the like. Use of C12A7 electride makes it unnecessary to workin dry or inactive gas during the formation of the organic EL element130. The conditions for the formation of the organic EL element 130 arealleviated.

C12A7 electride has a large ionization potential. Therefore, theelectron injection layer 150, when being located to face the holetransfer layer 154 while having the light emitting layer 152 between theelectron injection layer 150 and the hole injection layer 154therebetween, acts as a hole block layer. Namely, the electron injectionlayer 150 formed of C12A7 electride is provided between the electrontransfer layer 148 and the light emitting layer 152, so that holesinjected into the light emitting layer 152 are suppressed from runningto the first 146 as the cathode electrode, and thus the light emissonefficiency is improved.

2-2-4-4. Light Emitting Layer

The light emitting layer 152 may be formed of any of various materials.For example, the light emitting layer 152 may be formed a fluorescentcompound emitting fluorescence or phosphorescent compound emittingphosphorescence.

Examples of light emitting material emitting blue light usable for thelight emitting layer 152 include

-   N,N′-bis[4-(9H-carbazole-9-yl)phenyl-N,N′-diphenylstilbene-4,4′-diamine    (YGAS2S),    4-(9H-carbazole-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine    (YGAPA), and the like. Examples of light emitting material emitting    green light usable for the light emitting layer 152 include-   N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazole-3-amine    (2PCAPA),    N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazole-3-amine    (2PCABPhA),-   N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine    (2DPAPA),    N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine    (2DPABPhA),    N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazole-9-yl)phenyl]-N-phenylanthracene-2-amine    (2YGABPhA), N,N,9-triphenylanthracene-9-amine (DPhAPhA), and the    like. Examples of light emitting material emitting red light usable    for the light emitting layer 152 include-   N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (p-mPhTD),    7,13-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1.2-a]fluoranthene-3,10-diamine    (p-mPhAFD), and the like. A phosphorescent material such as    bis[2-(2′-benzo[4,5-α]thienyl)pyridinatho-N,C^(3′)]iridium(III)acetylacetonate    (Ir(btp)₂(acac)) or the like is also usable.

The light emitting layer 152 may be formed of any of various knownmaterials other than the above-listed materials. The light emittinglayer 152 may be formed by vapor deposition, transfer, spin-coating,spray-coating, gravure printing, or the like. The light emitting layer152 may have an optionally selected thickness, and has a thickness of,for example, 10 nm to 100 nm.

2-2-4-5. Hole Transfer Layer

The hole transfer layer 154 is formed of a material having holetransferability. The hole transfer layer 154 is formed of, for example,an arylamine-based compound, an amine compound containing a carbozolegroup, an amine compound containing a fluorene derivative, or the like.Examples of materials usable for the hole transfer layer 154 includeorganic materials such as

-   4,4′-bis[N-(naphtyl)-N-phenyl-amino]biphenyl (α-NPD),-   N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD),    2-TNANA,-   4,4′,4″-tris(N-(3-methylphenyl)N-phenylamino)triphenylamine    (MTDATA),-   4,4′-N,N′-dicarbazolebiphenyl (CBP),-   4,4′-bis[N-(9,9-dimethylfluorene-2-yl)-N-phenylamino]biphenyl    (DFLDPBi),-   4,4′bis[N-(spiro-9,9′-bifluorene-2-yl)-N-phenylamino]biphenyl    (BSPB), spiro-NPD, spiro-TPD, apiro-TAD, TNB, and the like.

The hole transfer layer 154 is formed of a common film formation methodsuch as vacuum vapor deposition, application or the like. The holetransfer layer 154 is formed by such a method to have a thickness of 10nm to 500 nm. The hole transfer layer 154 may be omitted.

2-2-4-6. Hole Injection Layer

The hole injection layer 156 is formed of a material having a highcapability of injecting holes into an organic layer. Examples ofmaterials having a high capability of injecting holes and usable for thehole injection layer 156 include metal oxides such as an oxide ofmolybdenum, an oxide of vanadium, an oxide of ruthenium, an oxide oftungsten, an oxide of manganese and the like. Examples of materialshaving a high capability of injecting holes and usable for the holeinjection layer 156 also include organic compounds such asphthalocyanine (H₂Pc), copper (II) phthalocyanine (CuPc),vanadylphthalocyanine (VoPc),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (TDATA),4,4′,4″-tris(N-(3-methylphenyl)-N-phenylamino]triphenylamine (MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (DPAB),

-   4,4′-bis(N-(4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl)-N-phenylamino)    biphenyl (DNPTD),-   1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (DPA3B),-   3-[N-(9-phenylcarbozole-3-yl)-N-phenylamino]-9-phenylcarbazole    (PCzPCA1),-   3,6-bis[N-(9-phenylcarbazole-3-yl)-N-phenylamino]-9-phenylcarbazole    (PCzPCA2),-   3-[N-(1-naphtyl)-N-(9-phenylcarbazole-3-yl)amino]-9-phenylcarbazole    (PCzPCN1), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene    (HAT-CN), and the like.

The hole injection layer 156 is formed of a common film formation methodsuch as vacuum vapor deposition, application or the like. The holeinjection layer 156 is formed by such a method to have a thickness of 1nm to 100 nm.

2-2-4-7. Anode Electrode

The second electrode 158 corresponding to the anode electrode is formedof a metal material, an alloy or a conductive compound having a highwork function (specifically, 4.0 eV or greater). The second electrode158 corresponding to the anode electrode is formed of, for example,indium tin oxide (ITO), indium zinc oxide (IZO), indium oxide containingtungsten oxide and zinc oxide (IWZO), or the like. The second electrode158 corresponding to the anode electrode of such a conductive metaloxide material is formed by vacuum vapor deposition or sputtering. Inthis embodiment, the organic EL element 130 is of a bottom emissiontype. Therefore, it is preferable that the second electrode 158corresponding to the anode electrode is light reflective or has a lightreflective surface. A film of a conductive metal oxide such as indiumtin oxide (ITO), indium zinc oxide (IZO) or the like islight-transmissive. Therefore, the second electrode 158 may include ametal film of aluminum (Al), silver (Ag) or the like at a surfacethereof that is opposite to a surface facing the hole injection layer156. Although not shown in FIG. 8, FIG. 9A and FIG. 9B, a passivationlayer blocking transmission of oxygen (O₂) or moisture (H₂O) may beprovided on the second electrode 158 corresponding to the anodeelectrode in substantially the entirety of the display region 121.

As described above, in this embodiment, the pixel 122 a in which thedriving transistor 126 exhibiting n-channel type conductivity and theorganic EL element 130 are electrically connected with each other isrealized. In this case, the organic EL element 130 may have an invertedstack structure, in which the electron transfer layer 148, the electroninjection layer 150, the light emitting layer 152, the hole transferlayer 154, the hole injection layer 156 and the like are stackedappropriately from the side of the first electrode 146, which is thecathode electrode. Since the first electrode 146, which is the cathodeelectrode, does not need to be formed of an alkaline metal material, thereliability of the display device 120 is improved. In addition, theelectron transfer layer 148 and the electron injection layer 150, whichare located in lower layers, are formed of an organic insulatingmaterial. Therefore, even if an organic layer is formed on theseinorganic insulating layers, the characteristics are suppressed fromdecreasing by denaturing or the like. Thus, the characteristics of theorganic EL element are stabilized.

2-3. Transistor Structure

As shown in FIG. 9A and FIG. 9B, the pixel 122 a in this embodiment hasa structure in which the anode electrode 158 covers the entire surfacesof the driving transistor 126 and the selection transistor 124. Thedriving transistor 126 and the selection transistor 124 each have a dualgate structure; specifically, the oxide semiconductor layer 112, wherethe channel region is formed, is provided between the first gateelectrode 104 and the second electrode 116.

FIG. 10A is cross-sectional view of a bottom gate type transistor 300.The transistor 300 includes a gate electrode 304, a first electrode 306,a first transparent conductive layer 308 a, a second transparentconductive layer 308 b, a first line 310 a, a second line 310 b, anoxide semiconductor layer 312, a second insulating layer 314, aflattening layer 342, and an anode electrode 358, which are stacked on asubstrate 302. In the bottom gate type transistor 300, the back channelside (the side of the oxide semiconductor layer 312 facing the anodeelectrode 358) is likely to be influenced by the anode electrode 358.Specifically, the anode electrode 358 has a positive potential, and aninterface between the oxide semiconductor layer 312 and the secondinsulating layer 314 (back channel interface), and the anode electrode358, are separated from each other by an interval of about 3 μm to 5 μm.Therefore, positive charges are accumulated on the back channel side ofthe oxide semiconductor layer 312. When the positive charges are easilyaccumulated on the back channel side, there occurs a problem that thethreshold voltage of the transistor 300 is shifted to a negative side(the transistor 300 becomes a normally-off transistor).

This phenomenon will be described with reference to FIG. 10B based on amethod for driving a display device. While the display device is driven,as shown in FIG. 10B, a period Tg in which an on-voltage V_(gon) isapplied to the gate electrode 304 of the transistor 300 that drives theorganic EL element 300 is much shorter than one frame period Tf (Tf>Tg).In a period, other than the period in which a positive on-voltageV_(gon) is applied to the gate electrode 304 of the transistor 300,namely, in the period (Tf−Tg), a negative off-voltage V_(goff) isapplied to the gate electrode 304. To the anode electrode 358, apositive constant voltage (VDD) is kept applied. Therefore, charges inthe second insulating layer 314 and the flattening layer 342 are driftedby an electric field generated in the anode electrode 358 and the gateelectrode 304, and as a result, the positive charges are accumulated onthe back channel side.

In order to solve such an inconvenience, at is preferable that gateelectrodes are provided above and below the oxide semiconductor layer112 as in this embodiment. In this case, the second gate electrode 116is grounded to have a constant potential or is supplied with the samevoltage as that of the first electrode 104, so that the potential on theback channel side is stabilized.

FIG. 11A shows an embodiment of the transistor 100 a. In thisembodiment, the transistor 100 a has a structure in which the first gateelectrode 104 in a lower layer and the second gate electrode 116 in anupper layer each overlap both of the first transparent conductive layer108 a and the second transparent conductive layer 108 b, whichcorrespond to the source and drain electrodes. The first gate electrode104 having width W_(bottom) in a channel length direction overlaps eachof the first transparent conductive layer 108 a and the secondtransparent conductive layer 108 b by width W_(ov1). The second gateelectrode 116 having width W_(top) in the channel length directionoverlaps each of the first transparent conductive layer 108 a and thesecond transparent conductive layer 108 b by width W_(ov2). Since thefirst gate electrode 104 and the second gate electrode 116 eachpartially overlap both of the first transparent conductive layer 108 aand the second transparent conductive layer 108 b, the channel region inthe oxide semiconductor layer 112 is substantially blocked against theexternal electric field. Therefore, even if the anode electrode 158 islocated to cover the entire surface of the transistor 100 a, thetransistor 100 a is not influenced by the electric field of the anodeelectrode 158. Thus, the threshold voltage of the transistor 100 a isprevented from being changed along with time.

FIG. 11B shows an embodiment of the transistor 100 a. In thisembodiment, the transistor 100 a has a structure in which the secondgate electrode 116 in an upper layer overlaps both of the firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b corresponding to the source and drain electrodes, and thefirst gate electrode 104 overlaps neither the first transparentconductive layer 108 a nor the second transparent conductive layer 108b. The second gate electrode 116 having width W_(top) in the channellength direction overlaps each of the first transparent conductive layer108 a and the second transparent conductive layer 108 b by widthW_(ov2). By contrast, width W_(bottom) of the first gate electrode 104in the channel length direction is narrower than the interval betweenthe first transparent conductive layer 108 a and the second transparentconductive layer 108 b, and the first gate electrode 104 is offset bywidth W_(off) from the first transparent conductive layer 108 a and thesecond transparent conductive layer 108 b. Since at least the secondgate electrode 116 partially overlaps both of the first transparentconductive layer 108 a and the second transparent conductive layer 108b, the channel region in the oxide semiconductor layer 112 issubstantially blocked against the external electric field. Therefore,the threshold voltage of the transistor 100 a is prevented from beingchanged along with time. Specifically, the area size of the region wherethe second gate electrode 116 overlaps the oxide semiconductor layer 112is larger than the area size of the region where the first gateelectrode 104 overlaps the oxide semiconductor layer 112. For thisreason, the influence of the charges that may be accumulated on the backchannel side is blocked. In other words, the first gate electrode 104and the second gate electrode 116 overlap each other as seen in a planview, and the second gate electrode 116 covers the first gate electrode104, and therefore, the influence of the charges that may be accumulatedon the back channel side is blocked.

In consideration of the alignment precision of a photomask inlithography step, it is preferable that width W_(top) of the second gateelectrode 116 is larger than width W_(bottom) of the first gateelectrode 104 (W_(top)>W_(bottom)). The width of the second gateelectrode 116 is made larger than the width of the first gate electrode104, so that there is a margin for the alignment precision of thephotomask in the lithography step. Therefore, the channel region formedin the oxide semiconductor layer 112 is covered with the second gateelectrode 116 with certainty.

2-4. Method for Producing the Display Device

An example of method for producing the display device 120 in anembodiment according to the present invention will be described. In thefollowing description, the same explanations as those on the method forproducing the transistor 100 a provided in embodiment 1 will be omitted,and only the differences will be provided.

FIG. 12, FIG. 13A and FIG. 13B show a stage of forming the first gateelectrodes 104 a and 104 b, the first capacitance electrode 160 a andthe first common line 136 a on the substrate 102, and a stage of formingthe first insulating layer 106. FIG. 12 is a plan view of a regioncorresponding to one pixel 122 a. FIG. 13A is a cross-sectional viewtaken along line A1-A2 in FIG. 12, and FIG. 13B is a cross-sectionalview taken along line B1-B2 in FIG. 12.

As shorn in FIG. 12, FIG. 13A and FIG. 13B, the first common line 136 aand the first capacitance electrode 160 a are formed of the sameconductive film as that of the first gate electrodes 104 a and 104 b.Therefore, the first gate electrode 104 a and the gate signal line 132 aare formed as one continuous pattern formed of a conductive film in thesame layer. Similarly, the first common line 136 a and the firstcapacitance electrode 160 a are formed as one continuous pattern formedof a conductive film in the same layer.

The first insulating layer 106 is formed on the first gate electrodes104 a and 104 b, the first common line 136 a, and the first capacitanceelectrode 160 a. For example, the first insulating layer 106 is formedby stacking the first silicon nitride film 141 a and the first siliconoxide film 140 a from the substrate 102 side. The first silicon nitridefilm 141 a is formed by plasma CVD by use of gas such as SiH₄, NH₃, N₂or the like as source gas. The first silicon oxide film 140 a is alsoformed by plasma CVD by use of SiH₄, N₂O, Si(OC₂H₅)₄(tetraethoxysilane), Si(OCH₃)₄ (tetramethoxysilane), or the likeoptionally. The first insulating layer 106 is formed on substantiallythe entire surface of the substrate 102.

FIG. 14A and FIG. 14B show a stage where the second conductive film 107and the third conductive film 109 formed on the first insulating layer106, and the resist mask 207 a, the resist mask 207 b, a resist mask 207c and a resist mask 207 d are formed thereon by use of a multi-gradationphotomask. As shown in FIG. 14A, the first contact hole 117 a is formedin advance in the first insulating layer 106 in order to expose thefirst common line 136 a. The resist masks 207 a, 207 b, 207 c and 207 dare formed such that regions thereof corresponding to regions of thethird conductive film 109 that are to become the second common line 136b (FIG. 14A) and the data signal line 134 (FIG. 14B) are thicker thanthe remaining region. The second conductive 107 is formed of atransparent conductive material, and the third conductive film 109 isformed of a metal material.

FIG. 15, FIG. 16A and FIG. 16B show a state where the third conductivefilm 109 and the second conductive film 107 are etched by use the resistmasks 207 a, 207 b, 207 b and 207 d. FIG. 15 is a plan view of a regioncorresponding to one pixel 122 a. FIG. 16A is a cross-sectional viewtaken along line A1-A2 in FIG. 15, and FIG. 16B is a cross-sectionalview taken along line B1-B2 in FIG. 15.

The first transparent conductive layer 108 a, the second transparentconductive layer 108 b, the third transparent conductive layer 108 c andthe fourth transparent conductive layer 108 d are formed on the firstinsulating layer 106. The first transparent conductive layer 108 a andthe second transparent conductive layer 108 b are formed such that endsof the first transparent conductive layer 108 a and the secondtransparent conductive layer 108 b overlap the first gate electrode 104a while having the first insulating layer 106 between the firsttransparent conductive layer 180 a/the second transparent conductivelayer 108 b and the first gate electrode 104 a. The third transparentconductive layer 108 c and the fourth transparent conductive layer 108 dare formed such that ends of the third transparent conductive layer 108c and the fourth transparent conductive layer 108 d overlap the firstgate electrode 104 b while having the first insulating layer 106 betweenthe third transparent conductive layer 108 c/the fourth transparentconductive layer 108 d and the first gate electrode 104 b. The secondcommon line 136 b is formed on the second transparent conductive layer108 b. The second common line 136 b is formed on the surface of thesecond transparent conductive layer 108 b. In this state, the firstcommon line 136 a, the second transparent conductive layer 108 b and thesecond common layer 136 b are electrically connected with each other.

The second capacitance electrode 160 b is formed in contact with a topsurface of the fourth transparent conductive layer 108 d. The secondcapacitance electrode 160 b is located to at least partially overlap thefirst capacitance electrode 160 a while having the fourth transparentconductive layer 108 d and the first insulating layer 106 between thesecond capacitance electrode 160 b and the first capacitance electrode160 a. The capacitance element is formed in a region where the firstcapacitance electrode 160 a and the second capacitance electrode 160 boverlap each other while having the first insulating layer 106therebetween.

The data signal line 134 is formed of the third conductive film 109. Thedata signal line 134 is formed in contact with a top surface of thethird transparent conductive layer 108 c. In this state, the thirdtransparent conductive layer 108 c and the data signal line 134 areelectrically connected with each other. The third transparent conductivelayer 108 c is provided along the data signal line 134, and thus iselectrically connected with the data signal line 134 with certainty.

An end of the second common line 136 b is located inner to an end of thesecond transparent conductive layer 108 b. With such an arrangement,even though the second transparent conductive layer 108 b and the secondcommon line 136 b are stacked on each other, the ends thereof form astepped portion. Therefore, the step coverage of the oxide semiconductorlayer 112 and the second insulating layer 114 formed in a later stage isin a good state. Similarly, an end of the data signal line 134 islocated inner to an end of the third transparent conductive layer 108 c,and an end of the second capacitance electrode 160 b is located inner toan end of the fourth transparent conductive layer 108 d. Therefore, thestep coverage of the oxide semiconductor layer 112 and the secondinsulating layer 114, which are to be formed on these ends in a laterstage, are in a good state.

FIG. 17, FIG. 18A and FIG. 18B show a stage of forming the oxidesemiconductor layer 112, the second insulating layer 114 and the fourthconductive film 115. FIG. 17 is a plan view of a region corresponding toone pixel 122 a. FIG. 18A is a cross-sectional view taken along lineA1-A2 in FIG. 17, and FIG. 18B is a cross-sectional view taken alongline B1-B2 in FIG. 17.

The first oxide semiconductor layer 112 a is formed to coversubstantially the entire surfaces of the first transparent conductivelayer 108 a and the second transparent conductive layer 108 b. Thesecond oxide semiconductor layer 112 b is formed to cover substantiallythe entire surfaces of the third transparent conductive layer 108 c andthe fourth transparent conductive layer 108 d. The first oxidesemiconductor layer 112 a and the second oxide semiconductor layer 112 bare formed as follows. An oxide semiconductor film is formed bysputtering by use of an oxide semiconductor as a target, and issubjected to a lithography step. As a result, the first oxidesemiconductor layer 112 a and the second oxide semiconductor layer 112 bhaving a predetermined shape described above are formed. The first oxidesemiconductor layer 112 a is formed in contact with, and thus iselectrically connected with, the first transparent conductive layer 108a and the second transparent conductive layer 108 b. The second oxidesemiconductor layer 112 b is formed in contact with, and thus iselectrically connected with, the third transparent conductive layer 108c and the fourth transparent conductive layer 108 d.

The second insulating layer 114 is formed on the first oxidesemiconductor layer 112 a and the second oxide semiconductor layer 112b. The second insulating layer 114 is formed by, for example, stackingthe second silicon oxide film 140 b and the second silicon nitride film141 b in this order from the oxide semiconductor layer 112 side. As aresult, the first silicon oxide film 140 a is formed below the oxidesemiconductor layer 112, and the second silicon oxide film 140 b isformed above the oxide semiconductor layer 112. The oxide semiconductorlayer 112 is held between the oxide insulating films, and thus issuppressed from having a defect (donor level) caused thereto by oxygendeficiency.

It is desirable that the silicon oxide films 140 a and 140 b do not haveoxygen deficiency so as not to draw out oxygen from the first oxidesemiconductor layer 112 a. Rather, it is even preferable that thesilicon oxide films 140 a and 140 b contain an excessive amount ofoxygen. The second insulating layer 114, after being formed, isheat-treated at a temperature of 250° C. to 400° C., and thus oxygen isdiffused from the first silicon oxide film 140 a and the second siliconoxide film 140 b to the first oxide semiconductor 112 a and the secondoxide semiconductor layer 112 b. Since such heat treatment is performed,even if the oxide semiconductor layer 112 includes oxygen deficiency,the oxygen deficiency is compensated for by oxygen diffused from theoxide semiconductor layer 112, and the defect, which would become adonor level, is extinguished. Therefore, the resistance is increased.

In the second layer 114, the second contact hole 117 b is formed in aregion overlapping the second capacitance electrode 160 b. Then, thefourth conductive film 115 is formed. The fourth conductive film 114 isformed in substantially the same manner as the first conductive film103.

FIG. 19, FIG. 20A and FIG. 20B show a stage of forming the second gateelectrode 116. FIG. 19 is a plan view of a region corresponding to onepixel 122 a. FIG. 20A is a cross-sectional view taken along line A1-A2in FIG. 19, and FIG. 20B is a cross-sectional view taken along lineB1-B2 in FIG. 19.

The second gate electrode 116 is formed by a lithography step and anetching step performed on the fourth conductive film 115. The secondgate electrode 116 a is formed to include a region overlapping the firstgate electrode 104 a. The second gate electrode 116 b is formed toinclude a region overlapping the first gate electrode 104 b. As aresult, the driving transistor 126 and the selection transistor 124 areformed. The capacitance element 128 is electrically connected with thesecond gate electrode 116 a via the contact hole 117 b.

As shown in FIG. 21A and FIG. 21B, the flattening layer 142 is formed tobury the selection transistor 124, the driving transistor 126 and thecapacitance element 128. The flattening layer 142 is formed of anorganic resin material such as, for example, an acrylic resin, apolyimide resin, an epoxy resin, a polyamide resin or the like. In theflattening layer 142, the opening 144 is formed in a region overlappingthe first electrode 146, which is the cathode electrode, in order toexpose the first oxide semiconductor layer 112 a. In the case where theflattening layer 142 is formed of a photosensitive resin material, theopening 144 is formed by exposure to light by use of a photomask. Beforethe flattening layer 142 is formed, an opening is formed in advance inthe second insulating layer 114 in a region corresponding to the opening144. Alternatively, an opening that exposes the first oxidesemiconductor layer 112 a may be formed in the second insulating layer114 in the step of forming the opening 144 in the flattening layer 142.It is preferable that the opening 144 in the flattening layer 142 has atapered inner wall in order to allow the organic EL element 130 to beformed easily.

FIG. 22A and FIG. 22B show a stage of forming the electron transferlayer 148 and the electron injection layer 150. The electron transferlayer 148 is formed of a metal oxide material. Examples of the metaloxide material usable for the electron transfer layer 148 includesubstantially the same materials described in embodiment 1,specifically, a four-component oxide material, a three-component oxidematerial, a two-component oxide material, and a one-component oxidematerial. The electron transfer layer 148 is formed by sputtering by useof any of the above-listed materials as a sputtering target. Theelectron injection layer 150 is formed of C12A7 electride. The electroninjection layer 150 may be formed by sputtering by use of C12A7electride as a sputtering target. In this case, the sputtering may beperformed by use of at least one type of gas selected from the groupconsisting of He (helium), Ne (neon), N₂ (nitrogen), Ar (argon) NO(nitrogen monoxide), Kr (krypton), and Xe (xenon). The electron transferlayer 148 and the electron injection layer 150 are used commonly in aplurality of pixels 122 a, and therefore are formed on substantially theentirety of a region where the pixels 122 a are located.

Then, the light emitting layer 152, the hole transfer layer 154, thehole injection layer 156 and the second electrode 158 as the anodeelectrode are formed. As a result, the pixel structure shown in FIG. 9Aand FIG. 9B is formed. The light emitting layer 152 is formed ofdifferent light emitting materials for red pixels, green pixels and bluepixels. In the case where light emitted from the light emitting layer152 has a white light emission spectrum, the light emitting layer 152may be formed in substantially the entirety of the display region 121 asa layer common to all the pixels 122 a. The hole transfer layer 154 andthe hole injection layer 156 are each formed on substantially theentirety of the region where the pixels 122 a are located, as layerscommon to all the pixels 122 a. The second electrode 158 as the anodeelectrode is used as a common electrode to the pixels 122 a, andtherefore is formed on substantially the entirety of the region wherethe pixels 122 a.

According to the method for producing the display device 120 in thisembodiment, a multi-gradation photomask is used to decrease the numberof photomasks required to produce the display device 120. The use of themulti-gradation photomask allows a plurality patterns (the firsttransparent conductive layer 108 a, the second transparent conductivelayer 108 b, the third transparent conductive layer 108 c, the fourthtransparent conductive layer 108 d, the data signal line 134, the secondcommon line 136 b, and the like) to be formed by performing exposuremerely once. This increases the productivity of display devices 120 andalso decreases the production cost.

In this embodiment, both of the selection transistor 124 and the drivingtransistor 126 are of a dual gate type. The present invention is notlimited to this. For example, the selection transistor 124 may be a topgate type transistor with no first gate electrode 104 b. The pixelcircuit is not limited to having a configuration shown in FIG. 7. Thetransistors and the organic EL element in this embodiment are applicableto a pixel circuit including three transistors for one pixel.

Embodiment 3

In embodiment 3, another embodiment of the method for producing thedisplay device 120 according to the present invention will be described.In the following description, differences from embodiment 2 will bedescribed.

This method is substantially the same as in embodiment 2 until the stageof forming the second gate electrode 116. On the second gate electrode116, the flattening layer 142 is formed. FIG. 23A is a cross-sectionalstructure taken along line A1-A2 in FIG. 19, and shows the stage offorming the flattening layer 142.

In this embodiment, the flattening layer 142 is formed of an insulatingfilm having polarity. The flattening layer 142 is formed of, forexample, a straight chain-type fluorine organic material. Example of thestraight chain-type fluorine organic material usable for the flatteninglayer 142 include a fluroalkylsilane (FAS)-based material. Example ofthe fluroalkylsilane (FAS)-based material includeH,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS),tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorodilane (FOTS), and thelike.

The flattening lay 142 is formed of a straight chain-type fluorineorganic material, and thus has a liquid-repelling surface. In moredetail, the flattening layer 142 contains bipolar molecules and sidechains, and therefore, as schematically shown in FIG. 23A, negativecharges are generated at the surface of the flattening layer 142 becauseof a microlayer separation phenomenon. The opening 144 exposing thefirst oxide semiconductor layer 112 a is formed in the flattening layer142, and then, the electron transfer layer 148 is formed.

FIG. 23B shows a stage where the electron transfer layer 148 is formed.The electron transfer layer 148 is formed of a metal oxide material.Examples of the metal oxide material usable for the electron transferlayer 148 include substantially the same materials described inembodiment 1, specifically, a four-component oxide material, athree-component oxide material, a two-component oxide material, and aone-component oxide material. These oxide materials are each a type ofdegenerate semiconductor, and have n-type conductivity, with which themajor carrier is an electron.

Negative charges are generated at the surface of the flattening layer142 because of the microlayer separation phenomenon. As a result, aregion, of the electron transfer layer 148 having an n-typeconductivity, that is in contact with the surface of the flatteninglayer 148 is depleted. Such a depleted region 149 of the electrontransfer layer 148 becomes a high resistance region with almost nocarrier. By contrast, a region, of the electron transfer layer 148, thatis in contact with the oxide semiconductor layer 112 a, remains as it iswithout being depleted. The electron transfer layer 148 having an n-typeconductivity is formed in substantially the entirety of the displayregion 121. The depleted region 149 of the electron transfer layer 148is in positional correspondence with a region between the pixels. Thepixels adjacent to each other are insulated from each other by thedepleted region 149. Therefore, the level of leak current flowinglaterally via the electron transfer lays 148 is decreased.

Conventionally, it is considered that in order to produce an organic ELelement, a hole injection layer, a hole transfer layer, a light emittinglayer, an electron transfer layer, an electron injection layer and acathode electrode need to be continuously formed in vacuum without beingexposed to the air. By contrast, in this embodiment according to thepresent invention, the electron transfer layer 148 and the electroninjection layer 150 are formed of an oxide semiconductor, which isstable even when being exposed to the air. Thus, the conditions forproducing an organic EL element have a certain degree of freedom.Specifically, in this embodiment according to the present invention, theorganic EL element may be produced as follows. First, the electrontransfer layer 148 and the electron injection layer 150 are formed invacuum by use of a sputtering device. Then, after the pressure isreturned to the atmospheric pressure, the hole transfer layer 154, thehole injection layer 156 and the anode electrode 158 are formed byanother film formation device. Therefore, a device for producing theorganic EL element is prevented from being enlarged, and the linebalance of the production process is easily adjusted. This embodimentaccording to the present invention has various advantages that, forexample, the production adjustment is easily done while maintenance ofthe production device is performed, and that the production efficiencyis improved.

Embodiment 4

In embodiment 4, the pixel structure is different from that inembodiment 2. In the following description, differences from embodiment2 will be described.

4-1. Pixel Structure 1

FIG. 24A and FIG. 24B shows a structure of a pixel 122 b in a displaydevice in this embodiment. FIG. 24A shows a cross-sectional structuretaken along line A1-A2 in the plan view of FIG. 8, and FIG. 24B shows across-sectional structure taken along line B1-B2 in the plan view ofFIG. 8.

As shown in FIG. 24A, in the pixel 122 b, the electron transfer layer148 is individually provided in correspondence with each organic ELelement 130. In other words, the electron transfer layer 148 is notprovided in substantially the entirety of the display region 121, butone electron transfer layer 148 is individually provided for each pixel121 b. In this case, it is preferable that the electron transfer layer148 has an area size larger than that of the opening 144 and smallerthan the light emitting layer 152. Namely, it is preferable that an endof the electron transfer layer 148 is located outer to an end of theopening 144 and inner to an end of the light emitting layer 152.Therefore, as shown in FIG. 24B, the electron transfer layer 148 is notlocated in a region where the selection transistor 124 is provided.Since the electron transfer layer 148 is formed to be larger than theopening 144, the light emitting layer 152 is prevented from contactingthe oxide semiconductor layer 122 a. Since the electron transfer layer148 is formed to be smaller than the light emitting layer 152, the holetransfer layer 154 is prevented from contacting the electron transferlayer 148.

The electron transfer layer 148 is formed of a metal oxide material. Aresist mask is formed by a lithography step, and dry etching or wetetching is performed to easily form the electron transfer layer 148. Theelectron transfer layer 148 is formed of a metal oxide material andtherefore has an n-type conductivity. As shown in FIG. 24A, since theelectron transfer layer 148 is formed individually for each pixel 122 b,the level of leak current between the pixels 122 b is decreased. In thecase where the electron injection layer 150 is formed of C12A7electride, the electron injection layer 150 does not increase the levelof leak current between adjacent pixels 122 b because C12A7 electridehas a high resistance. Therefore, in the case of being formed of C12A7electride, the electron injection layer 150 may be provided insubstantially the entirety of the display region 121 without causing theproblem of leak current.

As described above, the electron transfer layer 148 is individuallyprovided for the organic EL element 130 in each pixel 122 b, so that theundesirable possibility that the leak current between the pixels 122 b(in other words, crosstalk) is generated is solved.

4-2. Pixel Structure 2

The two layers of the electron transfer layer 148 and the electroninjection layer 150 may be formed by a lithography step and an etchingstep. FIG. 25A and FIG. 25B shows a structure of the pixel 122 c in sucha case. FIG. 25A shows a cross-sectional structure taken along lineA1-A2 in the plan view of FIG. 8. FIG. 25B shows a cross-sectionalstructure taken along line B1-B2 in the plan view of FIG. 8.

In the case where the electron transfer layer 148 and the electroninjection layer 150 are individually provided for each pixel 122 b, itis preferable that the electron transfer layer 148 and the electroninjection layer 150 each have an area size smaller than that of thelight emitting layer 152. Namely, it is preferable that ends of theelectron transfer layer 148 and the electron infection layer 150 arelocated outer to the end of the opening 144 and inner to the end of thelight emitting layer 152. In other words, it is preferable that theelectron transfer layer 148 and the electron injection layer 150 eachhave such a size that is covered with the light emitting layer 152. Theelectron transfer layer 148 and the electron injection layer 150 areeach smaller than the light emitting layer 152, so that the holetransfer layer 154 is prevented from contacting the electron transferlayer 148.

The electron transfer layer 148 and the electron injection layer 150 areindividually provided for the organic EL element 130 in each pixel 122b, so that the undesirable possibility that the leak current between thepixels 122 b (in other words, crosstalk) is generated is solved.

In this embodiment, the electron transfer layer 148 and the electroninjection layer 150 are formed of an oxide semiconductor, and thus areallowed to be patterned by a lithography step so as to be providedindividually for the organic EL element 130 in each pixel 122 b. Namely,the generation of the leak current between the pixels 122 b (in otherwords, crosstalk) is prevented and the image quality of the displaydevice is improved merely by adding one photomask for the productionprocess.

Embodiment 5

In embodiment 5, one of, or both of, the electron transfer layer 148 andthe electron injection layer 150 are individually provided for eachpixel, but in a different form from that of embodiment 4. In thefollowing description, mainly differences from embodiment 4 will bedescribed.

5-1. Pixel Structure 1

FIG. 26 shows an example of planar structure of a pixel 122 c in thisembodiment. FIG. 27A shows a cross-sectional structure taken along lineA3-A4 in FIG. 26, and FIG. 27B shows a cross-sectional structure takenalong line B3-B4 in FIG. 26. FIG. 27A shows the cross-sectionalstructure of the driving transistor 126 and the organic EL element 130.FIG. 27B shows the cross-sectional structure of the selection transistor124 and the capacitance element 128. The following description will bemade with reference to FIG. 26, FIG. 27A and FIG. 27B when necessary.The plan view of the pixel 122 c shown in FIG. 26 omits the organic ELelement 130.

As shown in FIG. 26 and FIG. 27A, the electron transfer layer 148 is incontact with a top surface of the oxide semiconductor layer 112 a, andis individually provided for each of the pixels 122 c. Therefore, asshown in FIG. 27B, the electron transfer layer 148 is not provided in aregion where the selection transistor 124 is provided.

The electron transfer layer 148 is formed of an oxide semiconductor,like the oxide semiconductor layer 112 a. In this case, the oxidesemiconductor material used for the electron transfer layer 148 and theoxide semiconductor material used for the oxide semiconductor layer 112a are made different from each other, so that the electron transferlayer 148 is selectively processed on the oxide semiconductor layer 112a. Specifically, the electron transfer layer 148 is formed of an oxidesemiconductor that is etched faster than the oxide semiconductor for theoxide semiconductor layer 112 a, so the electron transfer layer 148 isselectively processed.

It is preferable that the electron transfer layer 148 is formed of, forexample, a zinc (Zn)-based oxide semiconductor not containing tin (Sn)(e.g., ZnSiO_(x), ZnMgO_(x), ZnGaO_(x), etc.) and the oxidesemiconductor layer 112 a is formed of, for example, a tin (Sn)-basedoxide semiconductor not containing zinc (Zn), magnesium (Mg) or the like(e.g., InGaSnO_(x), InWSnO_(x), InSiSnO_(x), etc.). In other words, itis preferable that the electron transfer layer 148 contains zinc oxideand at least one selected from silicon oxide, magnesium oxide andgallium oxide, and that the oxide semiconductor layer 112 a contains tinoxide, indium oxide, and at least one selected from gallium oxide,tungsten oxide and silicon oxide. With such an arrangement, the etchingrate on the oxide semiconductor used for the electron transfer layer 148is made different from the etching rate on the oxide semiconductor usedfor the oxide semiconductor layer 112 a, and thus the selection ratio ismade high. Specifically, the etching rate on the electron transfer layer148 is made higher than that on the oxide semiconductor layer 112 a. Thebandgap relationship between the electron transfer layer 148 and theoxide semiconductor layer 112 a is made optimal. Specifically, thebandgap of the electron transfer layer 148 is made larger than that ofthe oxide semiconductor layer 112 a. For example, in the case where thebandgap of the oxide semiconductor layer 112 a is 3.0 eV or greater, thebandgap of the electron transfer layer 148 is preferably greater than,or equal to, the bandgap of the oxide semiconductor layer 112 a, andmore preferably 3.4 eV or greater. In the case where the bandgap of theelectron transfer layer 148 is 3.4 eV or greater, blue light is notabsorbed and thus the reliability is improved.

Before the flattening layer 142 is formed, the electron transfer layer148 is formed by patterning performed by lithoraphy step and an etchingstep. Thus, the electron transfer layer 148 is formed by microscopicprocessing. Since the electron transfer layer 148 is individuallyprovided for each pixel, the level of leak current flowing betweenadjacent pixels is decreased to suppress the generation of crosstalklike in embodiment 4.

5-2. Pixel Structure 2

The two layers of the electron transfer layer 148 and the electroninjection layer 150 may be individually formed, for each of the pixels122 c, on the oxide semiconductor layer 112 a. FIG. 28A and FIG. 28Bshow the structure of the pixel 122 c in such a case. FIG. 28A shows across-sectional structure taken long line A3-A4 in the plan view of FIG.26, and FIG. 28B shows a cross-sectional structure taken long line B3-B4in the plan view of FIG. 26.

As shown it FIG. 28A, the electron transfer layer 148 and the electroninjection layer 150 are individually provided, for each pixel 122 c, onthe oxide semiconductor layer 112 a. Therefore, as FIG. 28B, neither theelectron transfer layer 148 nor the electron transfer layer 148 and theelectron injection layer 150 is provided in a region where the selectiontransistor 124 is provided. In this manner, the electron transfer layer148 and the electron injection layer 150 are individually provided forthe organic EL element in each pixel, so that the level of leak currentflowing between adjacent pixels is decreased to suppress the generationof crosstalk.

5-3-1. Production Method 1

A method for producing the display device in this embodiment will bedescribed with reference to the drawings. In the following description,mainly differences from embodiment 2 will be described.

FIG. 29, FIG. 30A and FIG. 30B show a stage where the gate electrodes104 (the first gate electrode 104 a and the second gate electrode 104b), the first insulating layer 160, the transparent conductive layers108 (the first transparent conductive layer 108 a through the fourthtransparent conductive layer 108 d), the data signal line 134, the firstcommon line 136 a, and the second common line 136 b are formed on thesubstrate 102, and a layer to become the oxide semiconductor layer 112and a layer to become the electron transfer layer 148 are formed onsubstantially the entire surface of the substrate 102. FIG. 29 is a planview on this stage (the first insulating layer 106, the layer to becomethe oxide semiconductor layer 112 and the layer to become the electrontransfer layer 148 formed on substantially the entire surface of thesubstrate 102 are omitted). FIG. 30A shows a cross-sectional structuretaken along line A3-A4 in FIG. 29, and FIG. 30B shows a cross-sectionalstructure taken along line B3-B4 in FIG. 29.

The layer to become the oxide semiconductor layer 112 and the layer tobecome the electron transfer layer 148 are formed of a metal oxidematerial, and thus may be continuously formed by use of a sputteringdevice. In this case, as described above, it is preferable that thelayer to become the oxide semiconductor layer 112 and the layer tobecome the electron transfer layer 148 are formed of different metaloxide materials (in other words, different oxide semiconductormaterials).

FIG. 31A and FIG. 31B show a stage of forming a photoresist film 205 onthe layer to become the oxide semiconductor layer 112 and the layer tobecome the electron transfer layer 148 and exposing the photoresist film205 by use of a photomask. As the photomask, the multi-gradationphotomask 201 having the multi-gradation pattern 203 is used like inembodiment 1. In this case, a middle exposed portion of themulti-gradation photomask 201 corresponds to the oxide semiconductorlayer 112, and the non-exposed portion of the multi-gradation photomask201 corresponds to the electron transfer layer 148.

Then, the photoresist mask 205 is developed to form a resist mask 207 aas shown in FIG. 32A and FIG. 32B. The resist mask 207 a has portions ofdifferent thickness. FIG. 32A shows a state where the resist mask 207 ais thicker in a portion corresponding to a region where the electrontransfer layer 148 is to be formed, and is thinner in a portioncorresponding to a region where the first oxide semiconductor layer 112a and the second oxide semiconductor layer 112 b are to be formed.

The layer to become the electron transfer layer 148 and the layer tobecome the oxide semiconductor layer 112 are etched by use of the resistmask 207 a. On this stage, the first oxide semiconductor layer 112 a andthe second oxide semiconductor layer 112 b are formed. The layer tobecome the electron transfer layer 148 remains in substantially the samepattern as that of the first oxide semiconductor layer 112 a and thesecond oxide semiconductor layer 112 b. After the etching, an ashingprocess is performed to remove the thinner portion of the resist mask207 a and thus to expose a surface of the layer to become the electrontransfer layer 148. FIG. 33A and FIG. 33B show the resist mask 207 aafter the ashing process. As shown in FIG. 33A, the resist mask 207 aremains while covering a part of the layer to become the electrontransfer layer 148.

Then, the exposed portion of the layer to become the electron transferlayer 148 is etched. As shown in FIG. 34 (plan view) and FIG. 35A andFIG. 35B (cross-sectional views), the electron transfer layer 148 isselectively formed on the first oxide semiconductor layer 112 a as aresult of the etching. As described above, the electron transfer layer148 is formed of a material having a higher etching rate than that ofthe material of the first oxide semiconductor layer 112 a. With such anarrangement, the electron transfer layer 148 is formed on the firstoxide semiconductor layer 112 a while the first oxide semiconductorlayer 112 a and the second oxide semiconductor layer 112 b remainwithout being etched away. After the electron transfer layer 148 isformed by etching, the resist mask 207 a is removed by an ashingprocess.

Then, as shown in FIG. 36A and FIG. 36B, the second insulating layer114, the second gate electrodes 116 a and 116 b, and the flatteninglayer 142 are formed. In the flattening layer 142, the opening 144 isformed, and the electron injection layer 150 is formed. As shown in FIG.36A and FIG. 36B, the electron injection layer 150 is formed on the topsurface of the flattening layer 150 and in the opening 144 and is incontact with the electron transfer layer 148.

After this, the light emitting layer 152, the hole transfer layer 154,the hole injection layer 156 and the anode electrode 168 are formed. Asa result, the pixel 122 c of the display device shown in FIG. 26, FIG.27A and FIG. 27B is formed.

In this embodiment, the multi-gradation photomask is used to form theelectron transfer layer 148 on the first oxide semiconductor layer 112 awithout increasing the number of photomasks. The positions of the oxidesemiconductor layer 112 a and the electron transfer layer 148 aredetermined by one photomask, and thus the oxide semiconductor layer 112a and the electron transfer layer 148 are both formed with highprecision even in the case where the pixels are microscopic. Also inthis embodiment, the number of photomasks and the number ofphotolithography steps do not need to be increased, and thus theproduction cost is suppressed from increasing.

5-3-2. Production Method 2

On the stage shown in FIG. 30A and FIG. 30B, the electron injectionlayer 150 may be stacked on the electron transfer layer 148 and themethod described in 5-3-1 may be performed. Specifically, themulti-gradation photomask may be used to form the electron transferlayer 148 and the electron injection layer 150 by etching. As a result,as shown in FIG. 37A and FIG. 37B, the electron transfer layer 148 andthe electron injection layer 150 may be selectively formed on the firstoxide semiconductor layer 112 a.

Then, the flattening layer 142 is formed, and the opening 144 is formedin the flattening layer 142. The light emitting layer 152, the holetransfer layer 154, the hole injection layer 156 and the anode electrode158 are formed. As a result, the pixel 122 c shown in FIG. 28A and FIG.28B is formed.

In this embodiment, like in the case described in 5-3-1, themulti-gradation photomask is used to provide the electron transfer layer148 and the electron injection layer 150 on the first oxidesemiconductor layer 112 a without increasing the number of photomasks.

Embodiment 6

In embodiment 6, an example of pixel of a display device of a so-calledtop emission type, in which light from the organic EL element 130 isoutput in a direction opposite to the substrate 102, will be described.In the following description, differences from embodiment 2 will bedescribed.

FIG. 38 is a plan view of a pixel 122 d of a display device 120 in thisembodiment. FIG. 39 shows a cross-sectional structure taken along lineA5-A6 in FIG. 38. The selection transistor 124, the driving transistor126, the capacitance element 128 and the organic EL element 130 in thepixel 122 d each have substantially the same structure at in embodiment2.

The pixel 122 d includes a reflective layer 162 in a region overlappingthe first electrode 146, which is a cathode electrode. The reflectivelayer 162 is provided below the first electrode 146 with the firstinsulating layer 106 being located between the reflective layer 162 andthe first electrode 146. The reflective layer 162 is formed in, forexample, the same layer as that of the first gate electrode 104. Namely,the reflective layer 162 is formed of the first conductive layer 103described in embodiment 2. The second electrode 158, which is an anodeelectrode, is formed of a transparent conductive material such as indiumtin oxide, indium zinc oxide or the like.

In this embodiment, the first electrode 146, which the cathodeelectrode, is formed of a transparent conductive material. Light emittedby the light emitting layer 152, except for a component thereofpropagating in the organic EL element 130 as wave guide light, radiatesat least toward the first electrode 146, which is the cathode electrode,and toward the second electrode 158, which is the anode electrode. Thelight directed from the light emitting layer 152 toward the firstelectrode 146, which is the cathode electrode, is transmitted throughthe first electrode 146, which is the cathode electrode, and the firstinsulating layer 106, and is reflected by the reflective layer 162. Thelight reflected by the reflective layer 162 is partially output from thesecond electrode 158, which is the anode electrode. In order to increasethe intensity of the output light, it is preferable that the reflectivelayer 162 includes a metal film of a material having a high reflectancesuch a (Al), silver (Ag) or the like at a surface thereof facing thefirst electrode 146, which is the cathode electrode.

In FIG. 38 and FIG. 39, an end of the reflective layer 162 is locatedinner to an end of the first electrode 146, which is the cathodeelectrode. This embodiment is not limited to this. The reflective layer162 is provided below the first electrode 146 with the first insulatinglayer 106 being located between the reflective layer 162 and the firstelectrode 146. Therefore, the reflective layer 162 may be wider than thefirst electrode 146, which is the cathode electrode. The reflectivelayer 162 may be provided to be continuous throughout the plurality ofpixels 122 d, unless the reflective layer 162 is in contact with thefirst gate electrode 104 or the gate signal line 132.

As described above, in this embodiment, the reflective layer 162 isprovided below the first electrode 146, which is the cathode electrode,and thus the display device 120 including the pixel 122 d of a topemission type is realized. In this case, the reflective layer 162 may beformed of the same conductive film as that of the first gate electrode104. Therefore, the number of the steps of the production process is notincreased.

Embodiment 7

In embodiment 7, the pixel structure is different from the inembodiment. 2. FIG. 40 is a plan view of a pixel 122 e. FIG. 41A shows across-sectional structure taken along line A7-A8 in FIG. 40, and FIG.41B shows a cross-sectional structure taken along line B5-B6 in FIG. 40.The pixel 122 e has substantially the same equivalent circuit as thatshown in FIG. 7. In the following description, differences fromembodiment 2 will be described.

In the pixel 122 e, the data signal line 134 and the second common line136 b are formed of the fourth conductive film 115 described inembodiment 2. Specifically, the data signal line 134 and the secondcommon line 136 b are provided on the second in layer 114.

The second common line 136 b is electrically connected with the firstcommon line 136 a via the first contact hole 117 a running through thefirst insulating layer 106 and the second insulating layer 114. Thesecond common line 136 b is in contact with the first oxidesemiconductor layer 112 a via a third contact hole 117 c running throughthe second insulating layer 114. The second common line 136 b is incontact with the first oxide semiconductor layer 112 a in a region wherethe first transparent conductive layer 108 a also overlaps the commonline 136 b and the first oxide semiconductor layer 112 a. This regioncorresponds to the source region of the driving transistor 126.Therefore, the second common line 136 b is electrically connected withthe first common line 136 a and the source region of the drivingtransistor 126.

The data signal line 134 is in contact with the second oxidesemiconductor layer 112 b in a fourth contact hole 117 d running throughthe second insulating layer 114. The data signal line 134 is in contactwith the second oxide semiconductor layer 112 b in a region where thethird transparent conductive layer 108 c also overlaps the data signalline 134 and the second oxide semiconductor layer 112 b. This regioncorresponds to the source or drain region of the selection transistor124. Therefore, the data signal line 134 is electrically connected withthe source drain region of the selection transistor 124.

The capacitance element 128 is formed in a region where the firstcapacitance electrode 160 a and the second capacitance electrode 160 boverlap each other while having the first insulating layer 106therebetween. In this embodiment, the second capacitance electrode 160 bis formed of a portion of the fourth transparent conductive layer 108 dand the second oxide semiconductor layer 112 a extending from theselection transistor 124 onto the first capacitance electrode 160 a. Inthis region, it is preferable that the second oxide semiconductor layer112 b has a low resistance so as to act as an electrode. In this region,the second oxide semiconductor layer 112 b may be removed. The secondgate electrode 116 a is in contact with the second capacitance electrode160 b via the second contact hole 117 b, and thus the driving transistor126 and the capacitance element 128 are electrically connected with eachother.

In this embodiment, the data signal line 134 and the second common line136 b are formed in the same conductive layer as that of the second gateelectrode 116. Therefore, in the pixel 122 e, the selection transistor124 and the data signal line 134 are electrically connected with eachother with certainty, the driving transistor 126 and the second commonline 136 b are electrically connected with each other with certainty,and the driving transistor 126 and the capacitance element 128 areelectrically connected with each other with certainty.

Embodiment 8

In embodiment 8, an example of pixel of a display device of a so-calledtop emission type, in which light from the organic EL element 130 isoutput in a direction opposite to the substrate 102, will be described.In the following description, differences from embodiment 4 will bedescribed.

FIG. 42 is a plan view of a pixel 122 f of a display device 120 in thisembodiment. FIG. 43 shows a cross-sectional structure taken along lineA9-A10 in FIG. 42. The selection transistor 124, the driving transistor126, the capacitance element 128 and the organic EL element 130 in thepixel 122 f each have substantially the same structure as that inembodiment 4.

Like in embodiment 6, the pixel 122 f includes the reflective layer 162in a region overlapping the first electrode 146, which is a cathodeelectrode. The reflective layer 162 is provided below the firstelectrode 146 with the first insulating layer 106 being located betweenthe reflective layer 162 and the first electrode 146. The reflectivelayer 162 is formed in, for example, the same layer as that of the firstgate electrode 104. Namely, the reflective layer 162 is formed of thefirst conductive layer 103 described in embodiment 2. The secondelectrode 158, which is an anode electrode, is formed of a transparentconductive material such as indium tin oxide, indium zinc oxide or thelike.

The pixel 122 f of a top emission type may have a pixel structure inwhich the data signal line 134 and the second common line 136 b areformed in the same layer as that of the second gate electrode 116.

In FIG. 42 and FIG. 43, an end of the reflective layer 162 is locatedinner to an end of the first electrode 146, which is the cathodeelectrode. This embodiment is not limited to this. The reflective layer162 is provided below the first electrode 146 with the first insulatinglayer 106 being located between the reflective layer 162 and the firstelectrode 146. Therefore, the reflective layer 162 may be wider than thefirst electrode 146, which is the cathode electrode. The reflectivelayer 162 may be provided to be continuous throughout the plurality ofpixels 122 d, unless the reflective layer 162 is in contact with thefirst gate electrode 104 or the gate signal line 132.

In this embodiment, the reflective layer 162 is provided below the firstelectrode 146, which is the cathode electrode, and thus the displaydevice 120 including the pixel 122 f of a top emission type is realized.In this case, the reflective layer 162 may be formed of the sameconductive film as that of the first gate electrode 104. Therefore, thenumber of the steps of the production process is not increased.

Embodiment 9

In embodiment 9, the pixel structure is different from that inembodiment 4. FIG. 44 is a plan view of a pixel 122 g. FIG. 45A shows across-sectional structure taken along line A11-A12 in FIG. 44, and FIG.45B shows a cross-sectional structure taken along line B7-Ab8 in FIG.44. The pixel 122 g has substantially the same equivalent circuit asthat shown in FIG. 7. In the following description, differences fromembodiment 4 will be described.

In the pixel 122 g in this embodiment, the transparent conductive layer108 and the oxide semiconductor layer 122 are stacked in an orderdifferent from that in embodiment 4. Namely, the oxide semiconductorlayer 112 is provided on the first insulating layer 106, and thetransparent conductive layer 108 is provided on the oxide semiconductorlayer 112. Specifically, in the driving transistor 126, the firsttransparent conductive layer 108 a and the second transparent conductivelayer 108 b are provided on the first oxide semiconductor layer 112 a.The second transparent conductive layer 108 b is extended to the regionof the organic EL element 130, and acts as the first electrode 146,which is a cathode electrode, in this region. In the selectiontransistor 124, the third transparent conductive layer 108 c and thefourth transparent conductive layer 108 d are provided on the secondoxide semiconductor 112 b.

As can be seen, the transistors are realized even in the case where theorder of stacking the oxide semiconductor layer 112 and the transparentconductive layer 108 is changed. In this structure, in the case wherethe oxide semiconductor layer 112 and the transparent conductive layer108 have a certain level of etching rate ratio (in the case where theetching rate of the oxide semiconductor layer 112 is lower than that ofthe transparent conductive layer 108), the multi-gradation photomaskdescribed in embodiment 2 may be used to form the oxide semiconductorlayer 112 and the transparent conductive layer 108 by processing. Thus,the number of photomasks required for the production process isdecreased, and the steps of the production process is also decreased.

In this embodiment, as shown in FIG. 45A, the second common line 136 bis in contact with the second transparent conductive layer 108 b via thethird contact hole 117 c. As shown in FIG. 45B, the data signal line 134is in contact with the third transparent conductive layer 108 c via thefourth contact hole 117 d. As can be seen, the data signal line 134 andthe second common line 136 b are in contact with the transparentconductive layer 108, and thus the contact resistance is decreased.

Embodiment 10

FIG. 46 shows a stage in a method for forming a transistor 100 bincluding the first gate electrode 104, the first insulating layer 106,the oxide semiconductor layer 112, the second insulating layer 114 andthe second gate electrode 116. On the stage shown in FIG. 46, source anddrain regions 118 having a lower resistance than that of a channelregion is formed in the oxide semiconductor layer 112.

FIG. 46 shows a process of irradiating the oxide semiconductor layer 112with laser light from the substrate 102 side to form the source anddrain regions 118 having a resistance lower than that of the oxidesemiconductor layer 112. It is preferable that the laser light used inthis process has a short wavelength so as to be absorbed into the oxidesemiconductor layer 112, which has a wide bandgap. It is preferable thatthe oxide semiconductor layer 112 is irradiated with ultraviolet laserlight such as, for example, KrF excimer laser light (wavelength: 248nm), XeCl excimer laser light (wavelength: 308 nm), XeF excimer laserlight (wavelength: 351 nm) or the like.

In the case where the laser process shown in FIG. 46 is performed, thesubstrate 102 is required to have such a level of transparency as totransmit the ultraviolet laser light sufficiently. Therefore, it ispreferable that the substrate 102 is formed of non-alkali glass orquartz. In the case where the substrate 102 has a low transmittance forlight in an ultraviolet range or the substrate 102 absorbs the light inan ultraviolet range, the laser light may be directed from the oxidesemiconductor layer 112 side as shown in FIG. 47, so that a lowresistance region is formed.

In the case where the laser light is directed from the substrate 102side, a region of the oxide semiconductor layer 112 that overlaps thefirst gate electrode 104 is not irradiated with the laser light becausethe laser light is blocked by the first gate electrode 104. By contrast,the regions of the oxide semiconductor layer 112 not overlapping thefirst gate electrode 140 are irradiated with the laser light transmittedthrough the substrate 102. The irradiation with the laser light rapidlyraises the temperature of the oxide semiconductor layer 112. This causesoxygen deficiency, and thus a donor level is generated in the oxidesemiconductor layer 112. As a result, these regions have a resistancethereof decreased.

This process may be performed in a state where a contact hole isprovided in the second insulating layer 114 and a third line 110 c and afourth line 110 d are provided. The regions having the resistancethereof decreased act as a source region 118 a and a drain region 118 bof the transistor 110 b.

In the laser process shown in FIG. 46 performed to form the transistor110 b, the first gate electrode 104 is used as a mask blocking the laserlight. Thus, the source region 118 a and the drain region 118 b areformed in a self-aligned manner.

As shown in FIG. 47A, the laser process may be performed on a transistor110 c including the first line 110 a and the second line 110 b providedbetween the oxide semiconductor layer 112 and the second insulatinglayer 114 while being in contact with the oxide semiconductor layer 112.In this case also, the source region 118 a and the drain region 118 bare formed.

FIG. 47A shows an embodiment in which the oxide semiconductor layer 112is irradiated with the laser light from the second electrode 116 side.Regions of the oxide semiconductor layer 112 that overlap the secondelectrode 116, the first line 110 a and the second line 110 b are notirradiated with the laser light. The remaining region of the oxidesemiconductor layer 112 is irradiated with the laser light, and has theresistance thereof decreased. As shown in FIG. 47A, in a region of theoxide semiconductor layer 112 that is between a region thereofoverlapping the second gate electrode 116 and a region thereofoverlapping each of the first line 110 a and the second line 110 b, theresistance is decreased. In a region of the oxide semiconductor layer112 that is outer to the region thereof overlapping each of the firstline 110 a and the second line 110 b, the resistance is decreased.

In the structure shown in FIG. 47A, the source region 118 a and thedrain region 118 b are provided adjacent to channel region where theoxide semiconductor layer 112 is held between the first gate electrode104 and the second gate electrode 116. Namely, an offset region having ahigh resistance is not provided between the channel region and thesource region or between the channel region and the drain region.Therefore, the level of on-current is prevented from decreasing. Unlikein the transistor 100 a described in embodiment 1, it is not necessarythat the first line 110 a and the second line 110 b overlap each of thefirst electrode 104 and the second electrode 116. Therefore, thesource-gate coupling capacitance and the drain-gate coupling capacitanceare decreased.

FIG. 47B shows an embodiment in which the oxide semiconductor layer 112is irradiated with the laser light from the first gate electrode 104side. In this case, regions of the oxide semiconductor layer 112 that donot overlap the first gate electrode 104 are irradiated with the laserlight. As a result, regions of the oxide semiconductor layer 112 thatare below the first line 110 a and the second line 110 b also have aresistance thereof decreased. With such a process, the resistance of thesource region 118 a and the drain region 118 b is further decreased. Thecontact resistance between the source region 118 a and first line 110 a,and the contact resistance between the drain region 118 b and the secondline 110 b, are also decreased. Like in the structure shown in FIG. 46,the first gate electrode 104 is used as a mask, and the source region118 a and the drain region 118 b are formed in a self-aligned manner.Therefore, an offset region having a high resistance is not provided,and the level of on-current is prevented from decreasing.

As shown in FIG. 47A and FIG. 47B, the first gate electrode 104 or thesecond gate electrode 116 is used as mask blocking the laser light, sothat the source region 118 a and the drain region 118 b are formed inthe oxide semiconductor layer 112 in a self-aligned manner. Thisincreases the productivity of the integrated circuit elements includingthe transistor 100C and decreases the production cost.

As described in embodiment 2, in consideration of the alignmentprecision of the photomask, it is preferable that width W_(top) of thesecond gate electrode 116 is larger than width W_(bottom) of the firstgate electrode 104 (W_(top)>W_(bottom)). FIG. 48A shows an embodiment inwhich the oxide semiconductor layer 112 is irradiated with the laserlight from the second gate electrode 116 side in the case where thesecond gate electrode 116 is wider than the first gate electrode 104.

FIG. 48A shows an embodiment in which the laser light is directed fromthe second gate electrode 116 side in the case where width W_(top) ofthe second gate electrode 116 is larger than width W_(bottom) of thefirst gate electrode 104. A region of the oxide semiconductor layer thatis irradiated with the laser light has the resistance thereof decreased.In the case shown in FIG. 48A, the positions of the source region 118 aand the drain region 118 b adjacent to the channel region of the oxidesemiconductor layer 112 are determined by the second gate electrode 116.In other words, the source region 118 a and the drain region 118 b areformed in a self-aligned manner by the second gate electrode 116. Bycontrast, ends of the first gate electrode 104 does not match ends ofthe source region 180 a and the drain region 180 b, and offset regionseach having width W_(off) are present. However, the offset regions withrespect to the first gate electrode 104 are included in the channelregion for the second gate electrode 116. Therefore, the influence ofthe offset regions on static characteristics of the transistor 100C issmall.

FIG. 48B shows an embodiment in which the laser light is directed fromthe first gate electrode 104 side in the case where width W_(top) of thesecond gate electrode 116 is larger than width W_(bottom) of the firstgate electrode 104. In the case shown in FIG. 48B, the positions of thesource region 118 a and the drain region 118 b adjacent to the channelregion of the oxide semiconductor layer 112 are determined by the firstgate electrode 104. Therefore, the source region 118 a and the drainregion 118 b formed in the oxide semiconductor layer 112 include regionsoverlapping the second gate electrode 116. Such regions each have widthW_(ov). As can be seen, the source region 118 a and the drain region 118b have the regions overlapping the second gate electrode 116. Therefore,a high resistance region is not formed between the source region 118 aand the second gate electrode 116 or between the drain region 118 b andthe second gate electrode 118. Thus, the level of on-current of thetransistor 100 c is prevented from decreasing.

In this embodiment, the source region 118 a and the drain region 118 bare formed in a self-aligned manner by the first gate electrode 104 orthe second gate electrode 116. In consideration of the alignmentprecision in the lithography step, it is preferable that width W_(top)of the second gate electrode 116 is larger than width W_(bottom) of thefirst gate electrode 104 (W_(top)>W_(bottom)). Namely, the second gateelectrode 116 on the upper layer is made wider than the first gateelectrode 104 in the lower layer, so that there is a margin for thealignment precision of the photomask in the lithography step. Therefore,the channel region formed in the oxide semiconductor layer 112 iscovered with the second gate electrode 116 with certainty.

Embodiment 11

In embodiment 11, an example of applying the structure of the transistordescribed in embodiment 7 to a display device 120 will be described.

11-1. Pixel Structure 1

FIG. 49 is a plan view of a pixel 122 h of a display device 120 inembodiment 11. FIG. 50A shows a cross-sectional structure taken alongline A13-A14 in FIG. 49, and FIG. 50B shows a cross-sectional structuretaken along line B9-B10 in FIG. 49. The following description will bemade with reference to FIG. 49, FIG. 50A and FIG. 50B.

The first oxide semiconductor layer 112 a includes a region overlappingthe first gate electrode 104 a and the second gate electrode 116 a, andthe second oxide semiconductor layer 112 b includes a region overlappingthe first gate electrode 104 b and the second gate electrode 116 b. Thedriving transistor 126 includes the first source region 118 a and thefirst drain region 118 b provided in a region of the first oxidesemiconductor layer 112 a that is outer to the region thereofoverlapping the first gate electrode 104 a. The selection transistor 124includes a first source/drain region 118 c and a second source/drainregion 118 d provided in a region of the second oxide semiconductorlayer 112 b that is outer to the region thereof overlapping the firstgate electrode 104 b.

As described above in embodiment 7, the first source region 118 a, thefirst drain region 118 b, the first source/drain region 118 c and thesecond source/drain region 118 d are low-resistance regions generated asa result of being irradiated with laser light from the substrate 102side. The first gate electrodes 104 a and 104 b are used as masksagainst laser irradiation. Therefore, in each of the driving transistor126 and the selection transistor 124, regions of the oxide semiconductorlayer 112 other than the region thereof corresponding to the channelregion have resistance thereof decreased.

In the organic EL element 130, the regions of the first oxidesemiconductor layer 112 a having the resistance thereof decreased isused as first electrode 146, which is the cathode electrode. In thecapacitance element 128, the first capacitance electrode 160 a formed inthe same layer as that of the first gate electrode 104 overlaps thesecond oxide semiconductor layer 112 b. Therefore, the region of theoxide semiconductor layer 112 that overlaps the first capacitanceelectrode 160 a does not have a resistance thereof decreased. For thisreason, a wide opening is formed in the second insulating layer 114 toenlarge a region where the second gate electrode 116 a is in contactwith the second oxide semiconductor layer 112 b and overlaps the firstcapacitance electrode 160 a. With such an arrangement, the second gateelectrode 116 a also acts as the other capacitance electrode.

11-2. Pixel Structure 2

As described above in embodiment 4, the electron transfer layer 148 maybe individually provided for each pixel. FIG. 51, FIG. 52A and FIG. 52Bshow a structure of a pixel 122 h in this case. FIG. 51 is a plan viewof the pixel 122 h in a display 120 in this embodiment. FIG. 52A shows across-sectional structure taken along line A15-A16 in FIG. 51, and FIG.52B shows a cross-sectional structure taken along line B11-B12 in FIG.51.

As shown in FIG. 51, the opening 144 provided in the flattening layer142 exposes the inside of the first electrode 146 formed as a result ofthe resistance of the oxide semiconductor layer 112 being decreased. Asshown in FIG. 52A, the electron transfer layer 148 is in contact withthe first electrode 146 exposed by the opening 144. The electrontransfer layer 148 is individually provided for each pixel. For example,the electron transfer layer 148 is not provided in the entirety of thepixel 122 h. For example, as shown in FIG. 52B, the electron transferlayer 148 does not need to be provided in a region where the selectiontransistor 124 is provided. Like in embodiment 4, since the electrontransfer layer 148 is formed to be larger than the opening 144, thelight emitting layer 152 is prevented from contacting the oxidesemiconductor layer 112 a. Since the electron transfer layer 148 isformed to be smaller than the light emitting layer 152, the holetransfer layer 154 is prevented from contacting the electron transferlayer 148.

As shown in FIG. 52A, since an end of the electron transfer layer 148 islocated outer to the opening 144 (on the top surface of the flatteninglayer 142), the first electrode 146 formed of an oxide semiconductor isnot exposed outside. Namely, the first electrode 146 is not exposed tobe etched when the electron transfer layer 148 is formed by etching.Therefore, the first electrode 146 is not extinguished by over-etching.This allows the first electrode 146 (in other words, the oxidesemiconductor layer 112 a) to be formed to have the same thickness asthat of the channel region of the transistor.

11-3. Pixel Structure 3

As described above in embodiment 4, the two layers of the electrontransfer layer 148 and the electron injection layer 150 may be formed bya lithography step and an etching step. FIG. 53A and FIG. 53B show astructure of the pixel 122 h in such a case. FIG. 53A shows across-sectional structure taken along line A15-A16 in the plan view ofFIG. 51, and FIG. 53B shows a cross-sectional structure taken along lineB11-B12 in the plan view of FIG. 51.

As snow in FIG. 53A, both of the electron transfer layer 148 and theelectron injection layer 150 are provided in correspondence with theorganic EL element 130 in each pixel 122 h. In this case, as shown inFIG. 53B, neither electron transfer layer 148 nor the electron injectionlayer 150 is provided in a region where the selection transistor 124 isprovided. The electron transfer layer 148 and the electron injectionlayer 150 shown in FIG. 53A and FIG. 53B each have substantially thesame structure as that in embodiment 4.

As can be seen, in this embodiment, the driving transistor 126 and theselection transistor 124 included in the pixel 122 h both have a sourceregion and a drain region formed as a result of the resistance of theoxide semiconductor layer 112 being decreased. This simplifies thestructure of the transistors, and decreases the source-gate couplingcapacitance and the drain-gate coupling capacitance.

In this embodiment, the channel region of each of the driving transistor126 and the selection transistor 124, and the first electrode 146 actingas the cathode electrode of the organic EL element 130, are formed ofthe same oxide semiconductor layer 112. The source/drain regions 118 andthe second electrode 146 have resistances thereof decreased as a resultof being irradiated with laser light. This simplifies the productionprocess. In addition, since electron transfer layer 148 is individuallyformed for the organic EL element 130 in each pixel 122 h, theundesirable possibility that that the leak current between the pixels122 h (in other words, crosstalk) is generated is solved.

Embodiment 12 12-1. Pixel Structure

The pixel described in embodiment 11 may include a reflective electrode164 in the organic EL element 130. FIG. 54, FIG. 55A and FIG. 55B show astructure of a pixel 122 i including the reflective electrode 164provided between the first electrode 146 and the electron transfer layer148. FIG. 54 is a plan view showing a first structure of the pixel 122i. FIG. 55A shows a cross-sectional structure taken along line A17-A18in FIG. 54, and FIG. 55B shows a cross-sectional structure taken alongline B13-B14 in FIG. 54.

As shown in FIG. 54 and FIG. 55A, the reflective electrode 164 isprovided to be wider than the first electrode 146. An outer end of thereflective electrode 164 is located outer to the opening 144. As shownin FIG. 55A, the reflective electrode 164 is in contact with the firstelectrode 146. On the first electrode 146, the second insulating layer114 is provided. Therefore, the reflective electrode 164 is formed afterthe opening 144 a exposing the first electrode 146 is formed in thesecond insulating layer 114. The reflective electrode 164 may be formedof the same conductive layer as that of the second gate electrode 116(the common line 136 is also formed of the same conductive layer). Sincethe reflective electrode 164 is formed of the same conductive layer asthat of the second gate electrode 116 as described above, the number ofsteps of the production process is decreased. Namely, the reflectiveelectrode 164 is formed merely by forming the opening 144 a in thesecond insulating layer 114 without adding any other step.

In the case where the pixel 112 h is of a top emission type, in whichlight from the organic EL element 130 is output from the secondelectrode 158 side, it is preferable that the reflective electrode 164is provided on the first electrode 146 side. It is preferable that thereflective electrode 164 includes a metal layer having a highreflectance at a surface thereof facing the electron transfer layer 148.The reflective electrode 164 may include, for example, a first metallayer 166 a formed of aluminum (Al), an aluminum alloy, silver (Ag) orthe like. Examples of the aluminum alloy usable for the first metallayer 166 a include an aluminum-neodymium alloy (Al—Nd), analuminum-neodymium-nickel alloy (Al—Nd—Ni), an aluminum-carbon-nickel(Al—C—Ni), a copper-nickel alloy (Cu—Ni), and the like.

An aluminum film may undesirably cause an oxidation-reduction reactionwhen directly contacting the first electrode 146, which is formed of anoxide semiconductor. In order to avoid this, a second metal layer 166 bformed of a metal material such as titanium (Ti), tantalum (Ta),molybdenum (Mo) or the like may be provided between the first electrode146 and the first metal layer 166 a.

On the reflective electrode 164, the flattening layer 142 is provided. Asurface of the reflective electrode 164 is exposed by an opening 144 bformed in the flattening layer 142. The electron transfer layer 148 isin contact with the reflective electrode 164 via the opening 144 bformed in the flattening layer 142. A thin film formed of analuminum-lithium alloy (AlLi), a magnesium-silver alloy (MgAg) or thelike may be provided between the reflective electrode 164 and theelectron transfer layer 148.

12-2. Pixel Structure 2

FIG. 56A and FIG. 56B show a second structure of the pixel 122 i. FIG.56A shows a cross-sectional structure taken along line A17-A18 in theplan view of FIG. 54, and FIG. 56B shows a cross-sectional structuretaken along line B13-B14 in the plan view of FIG. 54. In the secondstructure, the electron transfer layer 148 and the electron injectionlayer 150 are individually provided for each pixel 122 i, like inembodiment 11. Even in the case where the electron transfer layer 148and the electron injection layer 150 are individually provided for eachpixel 122 i, the organic EL element 130 may include the reflectiveelectrode 164.

12-3. Pixel Structure 3

FIG. 57 is a plan view showing a third structure of the pixel 122 i.FIG. 58A shows a cross-sectional structure taken along line A19-A20 inFIG. 57, and FIG. 58B shows a cross-sectional structure taken along lineB15-B16 in FIG. 57. In the third structure, the reflective electrode 164is provided below the second insulating layer 114. In this case, thereflective electrode 164 is formed as follows. The oxide semiconductorlayer 112, the first metal layer 166 a and the second metal layer 166 bare formed on substantially the entire surface of the substrate 102, andthe reflective electrode 164 is formed by use of a multi-gradationphotomask as described above in embodiment 5.

FIG. 62A and FIG. 62B show an embodiment in which the electron transferlayer 148 and the electron injection layer 150 are individually providedfor each pixel 122 i, like in embodiment 11, in the third structure.Even in the case where the electron transfer layer 148 and the electroninjection layer 150 are individually provided for each pixel 122 i, theorganic EL element 130 may include the reflective electrode 164.

As can seen, in this embodiment, the reflective electrode 164 isprovided in the organic EL element 130, so that the intensity of thelight output from the top emission type pixel is increased. Namely, thecurrent efficiency of the organic EL element is increased. In this case,the reflective electrode 164 is formed of the same conductive layer asthat of the second gate electrode 116. Therefore, the number of steps ofthe production process does not need to be significantly increased toform the reflective electrode 164 in the organic EL element 130.

Embodiment 13

In embodiment 13, the electron transfer layer 148 is provided on theoxide semiconductor layer 112 a like in embodiment 5. FIG. 59 is a planview of a pixel 122 j in this embodiment. FIG. 60A shows across-sectional structure taken along line A21-A22 in FIG. 59, and FIG.60B shows a cross-sectional structure taken along line B17-B18 in FIG.59.

The first electrode 146 in contact with the electron transfer layer 148is provided in the same layer as that of the oxide semiconductor layer112 a included in the driving transistor 126, and has the resistancethereof decreased as a result of being irradiated with laser light fromthe substrate 102 side. The electron transfer layer 146 is formed by useof a multi-gradation photomask as in the steps shown in FIG. 31A, FIG.31B, FIG. 32A, FIG. 32B, FIG. 33A, FIG. 33B, FIG. 34, FIG. 35A, FIG.35B, FIG. 36A and FIG. 36B in embodiment 5.

FIG. 61A and FIG. 61B show a cross-sectional structure of the pixel 122j. In the structure shown in FIG. 61A and FIG. 61B, the electrontransfer layer 148 and the electron injection layer 150 provided on thefirst electrode 146 are formed by use of a multi-gradation photomask,unlike in the structure shown in FIG. 60A and FIG. 60B.

The first electrode 146 formed in the oxide semiconductor layer 122 hasthe resistance thereof decreased as a result of being irradiated withlaser light in the ultraviolet range from the substrate 102 side. Thisdecreases the contact resistance between the first electrode 146 and theelectron transfer layer 148, and thus a good ohmic junction is formed.

It is preferable that the oxide semiconductor layer 112 a and the firstelectrode 146 are formed of a tin (Sn)-based oxide semiconductor notcontaining zinc (Zn), magnesium (Mg) or the like (e.g., InGaSnO_(x),InWSnO_(x), InSiSnO_(x), etc.), and that the electron transfer layer 148is formed of a zinc (Zn)-based oxide semiconductor not containing tin(Sn) (e.g., ZnSiO_(x), ZnMgO_(x), ZnGaO_(x), etc.). With such anarrangement, the etching rate on the oxide semiconductor used for theelectron transfer layer 148 is made different from the etching rate onthe oxide semiconductor used for the oxide semiconductor layer 112 a,and thus the selection ratio is increased. In addition, the bandgaprelationship of the oxide semiconductor layer 112 a and the firstelectrode 146 with respect to the electron transfer layer 148 is madeoptimal. Specifically, the bandgap of the electron transfer layer 148 ismade larger than that of the oxide semiconductor layer 112 a and thefirst electrode 146. For example, the bandgap of the electron transferlayer 148 is preferably 3.4 eV or greater. In the case where the bandgapof the electron transfer layer 148 is 3.4 eV or greater, blue light isnot absorbed and thus the reliability is improved.

The entirety of, or a part of, the illustrative embodiments disclosedabove may be defined by the following supplementary Notes. Anyembodiment of the present invention is not limited to any of thefollowing.

Supplementary Note 1.

A display device includes a substrate; and a plurality of pixelsprovided on the substrate. The plurality of pixels each include adriving transistor and an organic EL element electrically connected withthe driving transistor; the driving transistor includes: an oxidesemiconductor layer; a first gate electrode including a regionoverlapping the oxide semiconductor layer, the first gate electrodebeing provided on a surface of the oxide semiconductor layer facing thesubstrate; a first insulating layer provided between the first gateelectrode and the oxide semiconductor layer; a second gate electrodeincluding a region overlapping the oxide semiconductor layer and thefirst gate electrode, the second gate electrode being provided on asurface of the oxide semiconductor layer opposite to the surface facingthe substrate; and a second insulating layer provided between the secondgate electrode and the oxide semiconductor layer; and the organic ELelement includes; a light-transmissive first electrode; a secondelectrode provided to face the first electrode; a light emitting layerprovided between the first electrode and the second electrode; and anelectron transfer layer provided between the light emitting layer andthe first electrode; wherein the first electrode is continuous from thefirst transparent conductive layer.

Supplementary Note 2.

The display device according to supplementary Note 1, wherein the secondgate electrode has a width in a channel length direction larger than awidth of the first gate electrode in the channel length direction.

Supplementary Note 3.

The display device according to supplementary Note 1, wherein the regionof the second gate electrode overlapping the oxide semiconductor layerhas an area size larger than an area size of the region of the firstgate electrode overlapping the oxide semiconductor layer.

Supplementary Note 4.

The display device according to supplementary Note 1, wherein theelectron transfer layer has a carrier concentration higher than acarrier concentration of the oxide semiconductor layer.

Supplementary Note 5.

The display device according to supplementary Note 1, wherein theelectron transfer layer has a bandgap of 3.4 eV or greater, and theoxide semiconductor layer has a bandgap of 3.0 eV or greater.

Supplementary Note 6.

The display device according to supplementary Note 1, wherein theorganic EL element further includes an electron injection layer providedbetween the electron transfer layer and the light emitting layer.

Supplementary Note 7.

The display device according to supplementary Note 6, wherein theelectron injection layer is formed of C12A7 (12CaO.7Al₂O₃) electride.

Supplementary Note 8.

The display device according to supplementary Note 1, wherein theelectron transfer layer is formed of a zinc (Zn)-based oxidesemiconductor not containing tin (Sn), and the oxide semiconductor layeris formed of a tin (Sn)-based oxide semiconductor containing neitherzinc (Zn) nor magnesium (Mg).

Supplementary Note 9.

The display device according to supplementary Note 8, wherein: theelectron transfer layer contains at least one selected from zinc oxide,silicon oxide, magnesium oxide and gallium oxide; and the oxidesemiconductor layer contains tin oxide, indium oxide and at least oneselected from gallium oxide, tungsten oxide and silicon oxide.

Supplementary Note 10.

The display device according to supplementary Note 1, further comprisinga flattening layer burying the driving transistor; wherein: theflattening layer has an opening exposing a top surface of the firstelectrode; and the electron transfer layer, the light emitting layer andthe second electrode are provided continuously on the top surface of thefirst electrode, on an inner wall of the opening and on a top surface ofthe flattening layer.

Supplementary Note 11.

The display device according to supplementary Note 10, wherein: theorganic EL element further includes a hole transfer layer and a holeinjection layer provided between the light emitting layer and the secondelectrode; the hole transfer layer and the hole injection layer areprovided continuously for the plurality of pixels; and the electrontransfer layer is individually provided for each of the plurality ofpixels.

Supplementary Note 12.

The display device according to supplementary Note 10, wherein:

the organic EL element further includes a hole transfer layer and a holeinjection layer provided between the light emitting layer and the secondelectrode; the hole transfer layer and the hole injection layer areprovided continuously for the plurality of pixels; and the electrontransfer layer and the electron injection layer are individuallyprovided for each of the plurality of pixels.

Supplementary Note 13.

The display device according to supplementary Note 11, wherein an end ofthe electron transfer layer is located outer to the opening and inner toan end of the light emitting layer.

Supplementary Note 14.

The display device according to supplementary Note 12, wherein an end ofthe electron transfer layer and an end of the electron injection layerare located outer to the opening and inner to an end of the lightemitting layer.

Supplementary Note 15.

The display device according to supplementary Note 1, further comprisinga reflective electrode provided between the first electrode and theelectron transfer layer.

Supplementary Note 16.

The display device according to supplementary Note 15, wherein thereflective electrode is provided in the same layer as that of the secondgate electrode.

What is claimed is:
 1. A transistor, comprising: an oxide semiconductorlayer over a substrate; a first gate electrode arranged on a side of theoxide semiconductor layer opposite to the substrate and including aregion overlapping the oxide semiconductor layer, a first insulatinglayer between the first gate electrode and the oxide semiconductorlayer; and a first transparent conductive layer and a second transparentconductive layer arranged on a side of the oxide semiconductor layer tothe substrate, wherein the first transparent conductive layer and thesecond transparent conductive layer are spaced apart from each other,the first gate electrode has a region overlapping a region where thefirst transparent conductive layer and the second transparent conductivelayer are spaced apart from each other, and the first transparentconductive layer and the second transparent conductive layer are incontact with the oxide semiconductor layer.
 2. The transistor accordingto claim 1, wherein a surface of the oxide semiconductor layer facingthe substrate has a first region in contact with the first transparentconductive layer and a second region in contact with the secondtransparent conductive layer, and wherein the first region is a sourceregion, and the second region is a drain region.
 3. The transistoraccording to claim 2, further comprising a first line arranged betweenthe first transparent conductive layer and the oxide semiconductorlayer, and a second line arranged between the second transparentconductive layer and the oxide semiconductor layer.
 4. The transistoraccording to claim 3, wherein the first line is electrically connectedto the oxide semiconductor layer and the first transparent conductivefilm, and the second line is electrically connected to the oxidesemiconductor layer and the second transparent conductive film.
 5. Thetransistor according to claim 4, wherein a first end of the firsttransparent conductive layer overlaps the first gate electrode, and afirst end of the second transparent conductive layer overlaps the firstgate electrode.
 6. The transistor according to claim 5, wherein thefirst line is arranged on a second end opposite to the first end of thefirst transparent conductive layer, and the second line is arranged on asecond end opposite to the first end of the second transparentconductive layer.
 7. The transistor according to claim 6, wherein theoxide semiconductor layer has a channel region overlapping the firstgate electrode, wherein the first end of the first transparentconductive layer is adjacent to the channel region, and the first end ofthe second transparent conductive layer is adjacent the channel region,and wherein the first line and the second line are arranged apart fromthe channel region.
 8. The transistor according to claim 7, wherein theoxide semiconductor layer includes low resistance regions having aresistivity lower than the channel region, and the low resistanceregions are adjacent to the channel region.
 9. The transistor accordingto claim 1, wherein the oxide semiconductor layer contains at least oneelement, or a plurality of elements selected from indium (In), zinc(Zn), gallium (Ga), tin (Sn), aluminum (Al), and magnesium (Mg).
 10. Thetransistor according to claim 1, wherein the first transparentconductive layer and the second transparent conductive layer areconductive metal oxide.
 11. The transistor according to claim 1, furthercomprising a second insulating layer between the first transparentconductive layer and the second transparent conductive layer and thesubstrate, wherein a second gate electrode between the second insulatinglayer and the substrate, and wherein the first gate electrode and thesecond gate electrode overlap each other.
 12. The transistor accordingto claim 11, wherein the second gate electrode has a length in a channellength direction larger than a length of the first gate electrode in thechannel length direction.
 13. The transistor according to claim 11,wherein an area of the second gate electrode overlapping the oxidesemiconductor layer has than an area of the first gate electrodeoverlapping the oxide semiconductor layer.
 14. The transistor accordingto claim 11, wherein the first gate electrode and the second gateelectrode ere electrically connected to each other.
 15. The transistoraccording to claim 11, wherein the second gate electrode is applied toconstant voltage.
 16. The transistor according to claim 11, wherein afirst end of the first transparent conductive layer overlaps the firstgate electrode and the second gate electrode, and a first end of thesecond transparent conductive layer overlaps the first gate electrodeand the second gate electrode.